Palladium(II) pyrazolin-4-ylidenes: Remote N-heterocyclic carbene complexes and their catalytic application in aqueous Suzuki-Miyaura coupling

Article abstract of DOI:10.1021/om7009107

Two monocationic complexes of pyrazole-derived remote carbene ligands of the type trans-[PdI(rNHC)(PPh₃)₂]⁺OTf ^- {rNHC = 2-ethyl-3,5-dim -1-phenylpyrazolin-4-ylidene (6a), 1-ethyl-2,3,5-trimethylpyrazolin-4-ylidene (6b)} were prepared by oxidative addition of 4-iodo-1,2,3,5-tetrasubstituted pyrazolium triflate salts to [Pd₂(dba)₃]/PPh₃ in good yields. Both compounds were fully characterized by multinuclei NMR spectroscopies, electrospray ionization mass spectrometry, and X-ray diffraction analysis. A comparative study on the aqueous Suzuki-Miyaura catalytic activities of these cationic complexes with their previously reported neutral counterparts reveals the superiority of the former.

Full text of DOI:10.1021/om7009107

Organometallics 2007, 26, 6581–6585
6581
Palladium(II) Pyrazolin-4-ylidenes: Remote N-Heterocyclic Carbene
Complexes and Their Catalytic Application in Aqueous
Suzuki-Miyaura Coupling
Yuan Han, Han Vinh Huynh,* and Geok Kheng Tan
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, Singapore
ReceiVed September 12, 2007
Two monocationic complexes of pyrazole-derived remote carbene ligands of the type trans-
[PdI(rNHC)(PPh3)2]+OTf– {rNHC ) 2-ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene (6a), 1-ethyl-2,3,5-
trimethylpyrazolin-4-ylidene (6b)} were prepared by oxidative addition of 4-iodo-1,2,3,5-tetrasubstituted
pyrazolium triflate salts to [Pd2(dba)3]/PPh3 in good yields. Both compounds were fully characterized by
multinuclei NMR spectroscopies, electrospray ionization mass spectrometry, and X-ray diffraction analysis.
A comparative study on the aqueous Suzuki-Miyaura catalytic activities of these cationic complexes
with their previously reported neutral counterparts reveals the superiority of the former.
Introduction
N-Heterocyclic carbenes (NHCs) have become “state of the
art” ligands in the area of organometallic chemistry and
transition-metal-mediated catalysis.1 In addition to the classical
C2-bound systems (“normal” carbenes, type A; Figure 1), NHC
Figure 1. Different types of NHCs: “normal” NHC (A); “abnormal”
NHC (B); rNHCs (C and D).
chemistry has recently been extended to complexes of “abnor-
mal” carbenes, which contain an unusual C4/C5 coordination
(type B).2 The latter contain a carbenoid center that is adjacent
to only one N heteroatom. More recently, Raubenheimer and
co-workers reported complexes bearing NHC ligands with a
6-membered ring, in which the carbenoid C is distant from the
heteroatom (type C). Computational studies on these remote
NHCs (rNHCs) derived from pyridine or quinoline suggested
an even stronger σ-donor ability compared to their well-known
normal NHC counterparts, which may be beneficial for certain
types of catalytic reactions.3 Indeed, preliminary catalytic studies
have shown that palladium(II) complexes with rNHC ligands
give rise to more active catalysts for some C-C coupling
reactions than precatalysts derived from standard NHCs.3a
Despite these promising properties, rNHCs have not attracted
the same degree of attention as common NHCs yet. As a
contribution to this little-explored field, we have recently
communicated the synthesis of the first neutral pyrazole-derived
rNHC (type D) complexes of palladium(II).4 Herein, we report
on the preparation, properties, and structural characterizations
of their monocationic counterparts. An initial study comparing
the catalytic activities of neutral and ionic palladium(II)
pyrazolin-4-ylidene complexes in the aqueous Suzuki-Miyaura
coupling reaction is included as well.
Results and Discussion
Palladium(II) Pyrazolin-4-ylidene Complexes. The syn-
thesis of suitable azolium precursors and the corresponding
neutral palladium(II) pyrazolin-4-ylidene complexes was com-
municated earlier.4 The ligand precursors 3a/b were prepared
in a short reaction sequence and transformed into the neutral
PdII(rNHC) complexes 4a/b by oxidative addition to [Pd2(dba)3]/
PPh3 (dba ) dibenzylideneacetone), as outlined in Scheme 1.
It is important to note that the in situ deprotonation of 1,2,3,5-
tetrasubstituted pyrazolium salts with basic metal precursors
such as Pd(OAc)2 or Ag2O was unsuccessful, probably because
of the low acidity of the pyrazolium C4 proton. Furthermore, it
is worth mentioning that premixing of [Pd2(dba)3] with PPh3
before the addition of the ligand precursors is important for
obtaining good yields of the complexes. For instance, complex
4b was obtained in a yield of only 45% when 3b, [Pd2(dba)3],
and PPh3 were added in a fast sequence.4 However, the yield
could be increased to 60% when the ligand precursor 3b was
added only after having mixed [Pd2(dba)3] and PPh3 in CH2Cl2
at ambient temperature for 10 min.
In an attempt to extend our research on pyrazolin-4-ylidenes,
we have also synthesized the monocationic PdII(rNHC) com-
plexes 6a/b according to Scheme 2. The 4-iodopyrazolium
triflate salt precursors 5a/b were easily obtained in quantitative
yields by reacting 3a/b with silver triflate. In a preparative
manner similar to that of 4a/b, treatment of 5a/b with premixed
[Pd2(dba)3] and PPh3 in CH2Cl2 under reflux conditions led to
the formation of the monocationic bis(phosphine) complexes
6a/b in good yields of 80 and 73%, respectively. Complexes
* E-mail: chmhhv@nus.edu.sg.
(1) (a) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348. (b) Kantchev,
E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46,
2768. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d)
César, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc. ReV. 2004, 33,
619. (e) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239. (f)
Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39.
(2) For a recent review, see Arnold, P. L.; Pearson, S. Coord. Chem.
ReV. 2007, 251, 596.
(3) (a) Schneider, S. K.; Roembke, P.; Julius, G. R.; Loschen, C.;
Raubenheimer, H. G.; Frenking, G.; Herrmann, W. A. Eur. J. Inorg. Chem.
2005, 2973. (b) Schneider, S. K.; Julius, G. R.; Loschen, C.; Raubenheimer,
H. G.; Frenking, G.; Herrmann, W. A. Dalton Trans. 2006, 1226.
(4) Han, Y.; Huynh, H. V. Chem. Commun. 2007, 1089.
10.1021/om7009107 CCC: $37.00
2007 American Chemical Society
Publication on Web 11/15/2007

6582 Organometallics, Vol. 26, No. 26, 2007
Han et al.
Scheme 1. Synthetic Pathway to Neutral PdII(rNHC)
Complexes
Scheme 2. Synthesis of the Ionic PdII(rNHC) Complexes
Figure 2. Molecular structure of the complex cation of
6a · CH2Cl2 · H2O showing 50% probability ellipsoids. The triflate
counterion, solvent molecules, and H atoms are omitted for clarity.
6a/b are stable in air and moisture. They are insoluble in
nonpolar solvents such as hexane, diethyl ether, and toluene
but generally more soluble than the neutral complexes 4a/b in
most polar solvents such as CHCl3, CH2Cl2, CH3CN, and
dimethyl sulfoxide (DMSO). The improved solubilities of 6a/b
allow for an identification of their carbene signals by 13C NMR
spectroscopy (vide infra).
The formation of complexes 6a/b was confirmed by electro-
spray ionization mass spectrometry (ESIMS), which shows base
peaks at m/z 957 (6a) and m/z 895 (6b) in their positive-mode
spectra, corresponding to the cationic complex [M-CF3SO3]+.
In the 31P NMR spectra, the phosphine donors in both complexes
give rise to very similar chemical shifts with values of 23.0
(6a) and 22.8 ppm (6b). The fact that only one 31P NMR signal
was observed for each complex indicates two equivalent
phosphine ligands and therefore a trans configuration of the
complex. The triflate 19F NMR resonances in 6a/b found in a
narrow range of -2.2 to -2.3 ppm remain largely unchanged
compared to those of the ligand precursors 5a/b, which is in
line with a noncoordinating triflate counteranion. In general,
the 1H NMR resonances of both complexes are shifted upfield
with respect to the corresponding signals of their ligand
precursors. More importantly, the 13C NMR signals for the
carbenoid C atoms in complexes 6a/b are observed at 128.2
and 127.8 ppm, respectively. Both signals appear as triplets
because of heteronuclear coupling with constants of 2J(C,P) )
7.3 Hz, again corroborating the trans arrangement of the two
phosphine ligands. Although these carbenoid resonances are
more upfield than the values commonly observed for analogous
palladium(II) complexes of other rNHCs5 or standard NHCs,3a,6
Figure 3. Molecular structure of the complex cation of 6b · CH2Cl2
showing 50% probability ellipsoids. The triflate counterion, solvent
molecules, H atoms, and one of the disordered C8 atoms are omitted
for clarity.
they are still significantly downfield-shifted by ∆δ ) 60.4 (6a)
and 61.0 ppm (6b) compared to the C4 carbon resonances in
their ligand precursors.
The identities of complexes 6a/b were further confirmed by
X-ray diffraction analyses on single crystals obtained from
concentrated CH2Cl2/Et2O solutions. The molecular structures
of the complex cations are depicted in Figures 2 and 3, selected
bond parameters are summarized in Table 1, and crystal-
lographic data are listed in Table 2. Both complexes crystallized
as solvates 6a · CH2Cl2 · H2O and 6b · CH2Cl2, respectively. As
found in solution NMR studies, both complexes exhibit the
expected trans configuration of the phosphine ligands in an
essentially square-planar geometry around the palladium center.
The carbene ring plane of the rNHC ligand in each complex is
situated almost perpendicularly to the PdICP2 coordination plane
with a torsion angle of 89.91° (6a) or 90.00° (6b) to relieve
steric congestion. These angles are bigger than those in the less
(5) (a) Meyer, W. H.; Deetlefs, M.; Pohlmann, M.; Scholz, R.;
Esterhuysen, M. W.; Julius, G. R.; Raubenheimer, H. G. Dalton Trans.
2004, 413. (b) Schuster, O.; Raubenheimer, H. G. Inorg. Chem. 2006, 45,
7997.
(6) Mathews, C. J.; Smith, P. J.; Welton, T.; White, A. J. P.; Williams,
D. J. Organometallics 2001, 20, 3848.

Palladium(II) Pyrazolin-4-ylidenes
Organometallics, Vol. 26, No. 26, 2007 6583
Table 1. Selected Bond Lengths [Å] and Angles [deg] for Complexes
6a/b
Table 3. Suzuki-Miyaura Cross-coupling Reactions Catalyzed by
4a/b and 6a/b in Aqueous Mediaa
6a · CH2Cl2 · H2O
6b · CH2Cl2
Pd1-C1
2.020(5)
2.6727(5)
2.3270(13)
2.3299(13)
1.391(7)
1.396(6)
1.351(6)
1.357(6)
1.369(6)
88.72(13)
89.37(13)
90.46(3)
91.27(3)
105.9(4)
89.91
2.004(9)
2.6685(9)
2.3375(15)
-
1.422(14)
1.374(15)
1.340(15)
1.379(15)
1.351(15)
86.84(5)
Pd1-I1
Pd1-P1
Pd1-P2
entry catalyst
aryl halide
t [h] T [°C] yield [%]b
C1-C2
C1-C3
1
2
3
4
5
6
7
8
4a
4b
6a
6b
4a
4b
6a
6b
4a
4b
6a
6b
4a
4b
6a
6b
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromoacetophenone
4-bromoacetophenone
4-bromoacetophenone
4-bromoacetophenone
4-bromoanisole
4-bromoanisole
4-bromoanisole
4-bromoanisole
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-chlorobenzaldehyde
19
19
19
19
19
19
19
19
21
21
21
21
21
21
21
21
RT
RT
RT
RT
RT
RT
RT
RT
80
80
80
80
80
80
82
93
92
80
67
95
91
N1-C2
N2-C3
N1-N2
C1-Pd1-P1
C1-Pd1-P2
P1-Pd1-I1
P2-Pd1-I1
C2-C1-C3
coordination plane/
carbene ring dihedral angle
93.20(4)
9
82c
90c
96c
92c
20c
14c
37c
39c
107.4(10)
90.00
10
11
12
13
14
15
16
Table 2. Selected X-ray Crystallographic Data for Complexes
80
80
80
6a · CH2Cl2 · H2O and 6b · CH2Cl2
6a · CH2Cl2 · H2O
6b · CH2Cl2
formula
fw
color, habit
cryst size [mm]
T [K]
C51H50Cl2F3IN2O4P2PdS C46H46Cl2F3IN2O3P2PdS
a Reaction conditions:
phenylboronic acid; 3 mL of water; 1.5 equiv of K2CO3; 1 mol % of
catalyst. b Yields were determined by 1H NMR spectroscopy for an
average of two runs. c With the addition of 1.5 equiv of [N(n-C4H9)4]Br.
1 mmol of aryl halide; 1.2 mmol of
1210.13
1130.05
yellow, thin plate
0.24 × 0.14 × 0.04
293(2)
colorless, plate
0.26 × 0.10 × 0.06
223(2)
cryst syst
space group
a [Å]
b [Å]
c [Å]
R [deg]
ꢀ [deg]
γ [deg]
V [Å3]
monoclinic
P2(1)/c
12.0615(5)
39.2897(18)
11.1728(5)
90
100.8010(10)
90
5200.9(4)
4
1.545
Mo KR
1.211
1.72–27.50
37279
orthorhombic
Pnma
10.0914(5)
22.4737(11)
20.9892(11)
90
90
90
4760.2(4)
4
1.577
Mo KR
1.315
1.81–27.50
32206
0.9253, 0.7262
R1 ) 0.0705,
wR2 ) 0.1551
R1 ) 0.1000,
wR2 ) 0.1674
1.079
standard test reaction, and the results are presented in Table 3.
We have chosen water as the reaction media because it is
economically and environmentally benign. In addition, it has
been shown that water can be a good solvent for the
Suzuki-Miyaura coupling reaction.7
From entries 1–8, it can be seen that all four complexes give
rise to catalysts that are able to couple the activated substrates
4-bromobenzaldehyde and 4-bromoacetophenone at ambient
temperature in moderate to very good yields. As for the
nonactivated substrate 4-bromoanisole, good conversions could
only be obtained at elevated temperatures and with the addition
of [N(n-C4H9)4]Br (TBAB) (entries 9–12). However, even under
such conditions, the coupling of the more difficult substrate
4-chlorobenzaldehyde afforded only low yields ranging from
14 to 39% (entries 13–16). Overall, reactions using the ionic
precatalysts 6a and 6b generally show remarkably better
conversions than those of their corresponding neutral counter-
parts 4a and 4b. This finding is consistent with the result
reported by Raubenheimer and co-workers, where the superiority
of the simple cationic NHC complex trans-[PdCl(Me2-imy)
(PPh3)2]+ over its corresponding neutral complex trans-[PdI2(Me2-
imy)(PPh3)] was observed in both Mizoroki-Heck and
Suzuki-Miyaura reactions.3a The catalyst derived from complex
6a exhibits the best catalytic activity in the reactions with aryl
bromides. However, in the coupling of 4-chlorobenzaldehyde,
6b performs slightly better.
Z
Dc [g cm-3
]
radiation used
µ [mm-1
]
θ range [deg]
unique data
max, min transmn 0.9532, 0.7599
final R indices
[I > 2σ(I)]
R1 ) 0.0607,
wR2 ) 0.1231
R indices (all data) R1 ) 0.0962,
wR2 ) 0.1356
GOF on F2
1.038
+1.558/-0.685
peak/hole [e Å-3
]
+1.245/-0.793
crowded complexes 4a/b (89.40° and 85.98°, respectively) for
obvious reasons. The Pd-P bonds amounting to 2.3270(13) and
2.3299(13) Å in 6a and 2.3375(15) Å in 6b are comparable to
those in analogous complexes containing other rNHCs.5 Fur-
thermore, both Pd-Ccarbene bond lengths of 2.020(5) Å (6a) and
2.004(9) Å (6b) fall within the normal range, with the former
slightly longer than the latter. The same trend was also observed
in the comparison of 4a [2.012(8) Å] with 4b [1.996(7) Å].
These observations suggest that 2-ethyl-3,5-dimethyl-1-phe-
nylpyrazolin-4-ylidene in 6a and 4a is a weaker ligand than
1-ethyl-2,3,5-trimethylpyrazolin-4 -ylidene in 6b and 4b because
of the less electron-donating N-phenyl substituent compared to
the N-methyl substituent as a consequence of different inductive
effects. However, the existence of such substituent effects in
pyrazole-based rNHCs remains to be investigated in more detail.
Catalysis. All pyrazolin-4-ylidene complexes discussed here
were subjected to Suzuki-Miyaura coupling reactions to test
their catalytic activities. The coupling of simple aryl bromides
and chlorides with phenylboronic acid in water under aerobic
conditions with 1 mol % catalyst loading was chosen as a
Conclusion
In conclusion, we have presented a straightforward synthetic
pathway to monocationic palladium(II) complexes containing
new pyrazole-derived rNHC ligands as an extension of NHC
chemistry. A preliminary study on rNHC-assisted palladium-
(7) (a) Bedford, R. B.; Blake, M. E.; Butts, C. P.; Holder, D. Chem.
Commun. 2003, 466. (b) Weng, Z.; Koh, L. L.; Hor, T. S. A. J. Organomet.
Chem. 2004, 689, 18. (c) Arvela, R. K.; Leadbeater, N. E. Org. Lett. 2005,
7, 2101. (d) DeVasher, R. B.; Moore, L. R.; Shaughnessy, K. H. J. Org.
Chem. 2004, 69, 7919. (e) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K.
Organometallics 2006, 25, 3267.

6584 Organometallics, Vol. 26, No. 26, 2007
Han et al.
(m, 3J(H,H) ) 7.2 Hz, 2 H, CH2), 4.03 (s, 3 H, NCH3), 2.49 (s, 3
H, CH3), 2.45 (s, 3 H, CH3), 1.30 (t, 3J(H,H) ) 7.2 Hz, 3 H,
CH2CH3). 13C{1H} NMR (75.47 MHz, DMSO-d6): δ 147.9, 146.8
(s, CCH3), 69.3 (s, CI), 43.2 (s, CH2), 35.1 (s, NCH3), 13.7, 13.2,
12.8 (s, CH3). Anal. Calcd for C8H14I2N2: C, 24.51; H, 3.60; N,
7.15. Found: C, 24.76; H, 3.80; N, 7.06. MS (ESI): m/z 265
[M - I]+.
catalyzed aqueous Suzuki-Miyaura coupling showed that the
novel cationic complexes give rise to more active catalysts than
their corresponding neutral counterparts. Research in our
laboratories is underway to extend the synthetic methodology
to other transition metals as well as to widen the scope of rNHC
complexes in catalysis.
cis-(2-Ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene)diiodo
(triphenylphosphine)palladium(II)(4a).Tris(dibenzylideneacetone)
dipalladium(0) (229 mg, 0.25 mmol) and triphenylphosphine (131
mg, 0.50 mmol) were dissolved in dry CH2Cl2 (30 mL) and stirred
at ambient temperature for 10 min under nitrogen. The resulting
dark-red solution was transferred to a suspension of 3a (227 mg,
0.5 mmol) in dry CH2Cl2 (20 mL) via cannula. The reaction mixture
was heated under reflux for 4 h in an inert nitrogen atmosphere
and then cooled to ambient temperature. The resulting mixture was
filtered through Celite, and the filtrate was extracted with H2O (4
× 30 mL). The CH2Cl2 layer was dried over MgSO4, and the solvent
was removed under reduced pressure. The residue was washed with
diethyl ether (2 × 30 mL) and dried in vacuo to give the crude
product as a yellow powder. Slow evaporation of a concentrated
CH2Cl2 solution at ambient temperature afforded the pure product
as yellow crystals (290 mg, 0.35 mmol, 70%). 1H NMR (300 MHz,
CD2Cl2): δ 7.78–6.87 (m, 20 H, Ar-H), 3.78 (m, 2J(H,H) ) 15.4
Experimental Section
General Considerations. Unless otherwise noted, all operations
were performed without taking precautions to exclude air and
moisture. All solvents and chemicals were used as received without
any further treatment if not noted otherwise. 1,3,5-Trimethylpyra-
zole was purchased from Sigma-Aldrich. Tris(dibenzylideneac-
etone)dipalladium(0) was received from Alfa Aesar. 1H, 13C, 31P,
and 19F NMR spectra were recorded on Bruker ACF 300 and AMX
500 spectrometers, and the chemical shifts (δ) were internally
referenced to the residual solvent signals relative to tetramethylsilane
(1H and 13C NMR) or externally to 85% H3PO4 (31P NMR) and
CF3CO2H (19F NMR). Mass spectra were measured using a
Finnigan MAT LCQ (ESI) spectrometer. Elemental analyses were
performed on a Perkin-Elmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
4-Iodo-3,5-dimethyl-1-phenylpyrazole (2a). An aqueous solu-
tion of KI3 [prepared by dissolving I2 (1505 mg, 5.95 mmol) and
KI (2955 mg, 17.8 mmol) in H2O (20 mL)] was added dropwise to
a solution of 3,5-dimethyl-1-phenylpyrazole (520 mg, 3 mmol) and
NaOAc (535 mg, 5.65 mmol) in H2O (12 mL) under reflux. The
resulting dark-brown solution was kept under reflux for another
7 h and then cooled to ambient temperature. Subsequently, an
aqueous solution of Na2S2O3 was added dropwise until the reaction
mixture was decolored. The product was extracted with diethyl ether
(4 × 30 mL). The combined organic layer was washed with
NaHCO3 (2 × 40 mL) and brine (2 × 40 mL). Drying over MgSO4
followed by removal of the solvent in vacuo afforded the product
3
2
Hz, J(H,H) ) 7.2 Hz, 1 H, CH2), 3.63 (m, J(H,H) ) 15.4 Hz,
3J(H,H) ) 7.2 Hz, 1 H, CH2), 2.36 (s, 3 H, CH3), 1.99 (s, 3 H,
3
CH3), 0.95 (m, J(H,H) ) 7.2 Hz, 3 H, CH2CH3). 31P{1H} NMR
(121 MHz, CD2Cl2): δ 29.4 (s, 1 P, PPh3). 13C{1H} NMR (75.47
3
MHz, CD2Cl2): δ 147.2 (d, J(P,C) ) 3.8 Hz, CCH3), 146.9 (d,
3J(P,C) ) 2.7 Hz, CCH3), 134.9 (d, 2/3J(P,C) ) 11.0 Hz, Ar-C),
132.9 (d, 1J(P,C) ) 48.3 Hz, Ar-C), 132.3, 131.6, 130.5 (s, Ar-C),
130.2 (d, 4J(P,C) ) 2.2 Hz, Ar-C), 130.1, 128.7, 127.8 (s, Ar-C),
127.7 (d, 2/3J(P,C) ) 11.0 Hz, Ar-C), 41.9 (s, CH2), 15.3, 15.1,
14.7(s, CH3), carbene signal not detected. Anal. Calcd for
C31H31I2N2PPd: C, 45.25; H, 3.80; N, 3.40. Found: C, 45.03; H,
4.28; N, 3.18. MS (ESI): m/z 695 [M - I]+.
1
as a brown oil (827 mg, 2.8 mmol, 93%). H NMR (300 MHz,
CDCl3): δ 7.48–7.33 (m, 5 H, Ar-H), 2.32 (s, 3 H, CH3), 2.30 (s,
3 H, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 150.8 (s, CCH3),
140.9, 140.0 (s, CCH3/Ar-C), 129.2, 128.0, 124.9 (s, Ar-C), 65.4
(s, CI), 14.2, 13.5 (s, CH3). Anal. Calcd for C11H11IN2: C, 44.32;
H, 3.72; N, 9.40. Found: C, 44.78; H, 3.78; N, 9.45. MS (ESI):
m/z 299 [M + H]+.
4-Iodo-1,3,5-trimethylpyrazole (2b). 2b was prepared analo-
gously to 2a from 1,3,5-trimethylpyrazole (441 mg, 4 mmol). The
product was purified by chromatography on silica using diethyl
ether. Yield: 637 mg, 2.7 mmol, 67%. 1H NMR (300 MHz, CDCl3):
δ 3.77 (s, 3 H, NCH3), 2.24 (s, 3 H, CH3), 2.19 (s, 3 H, CH3).
13C{1H} NMR (75.47 MHz, CDCl3): δ 149.0 (s, CCH3), 140.7 (s,
CCH3), 62.2 (s, CI), 37.1 (s, NCH3), 14.0, 12.0 (s, CH3). Anal.
Calcd for C6H9IN2: C, 30.53; H, 3.84; N, 11.87. Found: C, 30.89;
H, 3.76; N, 11.60. MS (ESI): m/z 237 [M + H]+.
cis-(1-Ethyl-2,3,5-trimethylpyrazolin-4-ylidene)diiodo(triph-
enylphosphine)palladium(II) (4b). 4b was prepared analogously
to 4a from 3b (196 mg, 0.5 mmol), tris(dibenzylideneacetone)di-
palladium(0) (229 mg, 0.25 mmol), and triphenylphosphine (131
1
mg, 0.50 mmol). Yield: 228 mg, 0.30 mmol, 60%. H NMR (500
3
MHz, CD2Cl2): δ 7.67–7.31 (m, 15 H, Ar-H), 3.86 (m, J(H,H)
) 7.3 Hz, 2 H, CH2), 3.43 (s, 3 H, NCH3), 2.21 (s, 3 H, CH3),
3
2.13 (s, 3 H, CH3), 1.15 (m, J(H,H) ) 7.3 Hz, 3 H, CH2CH3).
31P{1H} NMR (202 MHz, CD2Cl2): δ 29.3 (s, 1 P, PPh3). 13C{1H}
NMR (125.76 MHz, CD2Cl2): δ 145.9 (d, 3J(P,C) ) 4.6 Hz, CCH3),
145.4 (d, 3J(P,C) ) 3.7 Hz, CCH3), 135.0 (d, 2/3J(P,C) ) 11.0 Hz,
Ar-C), 132.7 (d, 1J(P,C) ) 48.6 Hz, Ar-C), 130.2 (d, 4J(P,C) )
1.8 Hz, Ar-C), 127.7 (d, 2/3J(P,C) ) 11.0 Hz, Ar-C), 41.6 (s,
CH2), 33.2 (s, NCH3), 15.0, (s, CH3), 14.9 (s, CH3), 14.7 (s, CH3),
carbene signal not detected. Anal. Calcd for C26H29I2N2PPd ·
CH2Cl2: C, 38.35; H, 3.69; N, 3.31. Found: C, 38.47; H, 3.85; N,
3.22. MS (ESI): m/z 633 [M - I]+.
2-Ethyl-4-iodo-3,5-dimethyl-1-phenylpyrazolium Iodide (3a). 2a
(713 mg, 2.4 mmol) was dissolved in iodoethane (2 mL) and heated
under reflux for 2 days shielded from light. The reaction mixture
was cooled to ambient temperature, and all volatiles were removed
under reduced pressure. The residue was washed with diethyl ether
and dried in vacuo to give the product as an off-white powder (545
2-Ethyl-4-iodo-3,5-dimethyl-1-phenylpyrazolium Triflate (5a).
3a (197 mg, 0.43 mmol) and silver triflate (118 mg, 0.46 mmol)
were suspended in CH3CN (5 mL) and stirred at ambient temper-
ature for 30 min shielded from light. The reaction mixture was
filtered through Celite, and the solvent of the filtrate was evaporated
off. To the residue was added CH2Cl2 (15 mL), and the resulting
mixture was filtered through Celite. Removal of the solvent in vacuo
afforded the product as an off-white powder (201 mg, 0.42 mmol,
98%). 1H NMR (300 MHz, CDCl3): δ 7.74–7.62 (m, 5 H, Ar-H),
4.29 (m, 3J(H,H) ) 7.3 Hz, 2 H, CH2), 2.62 (s, 3 H, CH3), 2.24 (s,
1
mg, 1.2 mmol, 50%). H NMR (300 MHz, CDCl3): δ 7.81–7.65
3
(m, 5 H, Ar-H), 4.39 (m, J(H,H) ) 7.3 Hz, 2 H, CH2), 2.70 (s,
3
3 H, CH3), 2.25 (s, 3 H, CH3), 1.22 (t, J(H,H) ) 7.3 Hz, 3 H,
CH2CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 149.9, 149.8 (s,
CCH3), 133.6, 131.8, 131.6, 129.8 (s, Ar-C), 72.6 (s, CI), 46.7 (s,
CH2), 15.4 (s, C CH3), 15.0 (s, CH2CH3). Anal. Calcd for
C13H16I2N2: C, 34.39; H, 3.55; N, 6.17. Found: C, 34.78; H, 3.45;
N, 6.19. MS (ESI): m/z 327 [M - I]+.
3
3 H, CH3), 1.20 (t, J(H,H) ) 7.3 Hz, 3 H, CH2CH3). 19F{1H}
1-Ethyl-4-iodo-2,3,5-trimethylpyrazolium Iodide (3b). 3b was
prepared analogously to 3a from 2b (283 mg, 1.2 mmol). Yield:
153 mg, 0.39 mmol, 33%. 1H NMR (300 MHz, DMSO-d6): δ 4.54
NMR (282 MHz, CDCl3): δ -2.3 (s, 3 F, CF3). 13C{1H} NMR
(75.47 MHz, CDCl3): δ 149.6, 149.5 (s, CCH3), 133.2, 131.5, 131.2,
129.0 (s, Ar-C), 120.8 (m, 1J(C,F) ) 320.7 Hz, CF3), 67.8 (s,

Palladium(II) Pyrazolin-4-ylidenes
Organometallics, Vol. 26, No. 26, 2007 6585
CI), 44.9 (s, CH2), 14.5, 14.2, 13.9 (s, CH3). Anal. Calcd for
C14H16F3IN2O3S: C, 35.31; H, 3.39; N, 5.88. Found: C, 35.16; H,
3.13; N, 5.71. MS (ESI): m/z 327 [M - CF3SO3]+.
(500 MHz, CDCl3): δ 7.55–7.36 (m, 30 H, Ar-H), 3.80 (m, 3J(H,H)
) 7.3 Hz, 2 H, CH2), 3.40 (s, 3 H, NCH3), 1.77 (s, 3 H, CH3),
1.73 (s, 3 H, CH3), 1.03 (t, 3J(H,H) ) 7.3 Hz, 3 H, CH2CH3).
31P{1H} NMR (202 MHz, CDCl3): δ 22.8 (s, 2 P, PPh3). 19F{1H}
NMR (282 MHz, CDCl3): δ -2.3 (s, 3 F, CF3). 13C{1H} NMR
(125.76 MHz, CDCl3): δ 144.7 (t, 3J(P,C) ) 3.2 Hz, CCH3), 143.9
(t, 3J(P,C) ) 3.7 Hz, CCH3), 134.7 (t, 2/3J(P,C) ) 6.4 Hz, Ar-C),
1-Ethyl-4-iodo-2,3,5-trimethylpyrazolium Triflate (5b). 5b was
prepared analogously to 5a from 3b (169 mg, 0.43 mmol) and silver
triflate (118 mg, 0.46 mmol). Yield: 174 mg, 0.42 mmol, 98%. 1H
NMR (300 MHz, CDCl3): δ 4.54 (m, 3J(H,H) ) 7.3 Hz, 2 H, CH2),
4.09 (s, 3 H, NCH3), 2.50 (s, 3 H, CH3), 2.49 (s, 3 H, CH3), 1.42
1
131.2 (t, J(P,C) ) 24.3 Hz, Ar-C), 131.1 (s, Ar-C), 128.4 (t,
3
2
(t, J(H,H) ) 7.3 Hz, 3 H, CH2CH3). 19F{1H} NMR (282 MHz,
2/3J(P,C) ) 5.1 Hz, Ar-C), 127.8 (t, J(P,C) ) 7.3 Hz, Ccarbene),
CDCl3): δ -2.5 (s, 3 F, CF3). 13C{1H} NMR (75.47 MHz, CDCl3):
121.0 (m, 1J(F,C) ) 320.8 Hz, CF3), 42.2 (s, CH2), 34.2 (s, NCH3),
14.9, 14.4, 14.0 (s, C CH3/CH2CH3). Anal. Calcd for C45H44IN2P2
PdO3SF3: C, 50.25; H, 4.17; N, 2.58. Found: C, 50.28; H, 4.07; N,
2.56. MS (ESI): m/z 895 [M - CF3SO3]+.
1
δ 148.7, 147.5 (s, CCH3), 120.7 (m, J(C,F) ) 320.2 Hz, CF3),
66.8 (s, CI), 44.4 (s, CH2), 35.7 (s, NCH3), 14.3 (s, CH3), 13.8 (s,
CH3), 13.5 (s, CH3). Anal. Calcd for C9H14F3IN2O3S: C, 26.10; H,
3.41; N, 6.76. Found: C, 26.08; H, 2.94; N, 6.62. MS (ESI): m/z
265 [M - CF3SO3]+.
General Procedure for the Suzuki-Miyaura Cross-coupling
Reaction. In a typical run, a test tube was charged with a mixture
of aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), potassium
carbonate (1.5 mmol), precatalyst (0.01 mmol) and [N(n-C4H9)4]Br
(1.5 mmol) (for entries 9–16 in Table 3). To the mixture was added
H2O (3 mL). The reaction mixture was vigorously stirred at the
appropriate temperature. After the desired reaction time, the solution
was allowed to cool. A total of 10 mL of dichloromethane was
added to the reaction mixture, and the organic phase was extracted
with water (6 × 5 mL) and dried over MgSO4. The solvent was
removed by evaporation to give a crude product, which was
trans-(2-Ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene)iodobis-
(triphenylphosphine)palladium(II) Triflate (6a). Tris(dibenzylide-
neacetone)dipalladium(0) (92 mg, 0.10 mmol) and triphenylphos-
phine (105 mg, 0.40 mmol) were dissolved in dry CH2Cl2 (20 mL)
and stirred at ambient temperature for 10 min under nitrogen. The
resulting dark-red solution was transferred to a solution of 5a (83
mg, 0.20 mmol) in dry CH2Cl2 (10 mL) via cannula. The reaction
mixture was heated under reflux for 4 h in an inert nitrogen
atmosphere and then cooled to ambient temperature. The resulting
mixture was filtered through Celite, and the filtrate was extracted
with H2O (4 × 30 mL). The CH2Cl2 layer was dried over MgSO4,
and the solvent was reduced under vacuum to 1 mL. Adding diethyl
ether to the concentrated solution resulted in an off-white precipitate,
which was collected and washed with diethyl ether again (2 × 30
mL) to give the product as a light-yellow powder (177 mg, 0.16
mmol, 80%). 1H NMR (500 MHz, CDCl3): δ 7.62–7.41 (m, 33 H,
Ar-H), 6.76–6.75 (m, 2 H, Ar-H), 3.62 (m, 3J(H,H) ) 7.3 Hz, 2
H, CH2), 2.07 (s, 3 H, CH3), 1.68 (s, 3 H, CH3), 0.76 (m, 3J(H,H)
) 7.3 Hz, 3 H, CH2CH3). 31P{1H} NMR (202 MHz, CDCl3): δ
23.0 (s, 2 P, PPh3). 19F{1H} NMR (282 MHz, CDCl3): δ -2.2 (s,
3 F, CF3). 13C{1H} NMR (125.76 MHz, CDCl3): δ 146.4 (t, 3J(P,C)
1
analyzed by H NMR spectroscopy.
X-ray Diffraction Studies. Diffraction data for complexes 6a/b
were collected with a Bruker AXS APEX CCD diffractometer
equipped with a rotation anode at 293(2) K (6a) or 223(2) K (6b)
using graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å).
Data were collected over the full sphere and were corrected for
absorption. Structure solutions were found by the Patterson method.
Structure refinement was carried out by full-matrix least squares
on F2 using SHELXL-978 with first isotropic and later anisotropic
displacement parameters for all non-H atoms. A summary of the
most important crystallographic data is given in Table 2.
3
) 3.2 Hz, CCH3), 145.1 (t, J(P,C) ) 3.2 Hz, CCH3), 134.8 (t,
Acknowledgment. We thank the National University of
Singapore for financial support (Grant No. R 143-000-268-
112) and the CMMAC staff of our department for technical
assistance.
1
2/3J(P,C) ) 6.0 Hz, Ar-C), 132.3 (s, Ar-C), 131.7 (t, J(P,C) )
24.7 Hz, Ar-C), 131.3, 131.2, 130.8 (s, Ar-C), 128.6 (t, 2/3J(P,C)
2
) 5.5 Hz, Ar-C), 128.2 (t, J(P,C) ) 7.3 Hz, Ccarbene), 127.9 (s,
1
Ar-C), 121.0 (m, J(F,C) ) 320.8 Hz, CF3), 42.8 (s, CH2), 14.8,
14.7, 14.6 (s, CH3). Anal. Calcd for C50H46F3IN2O3P2PdS: C, 54.24;
H, 4.19; N, 2.53. Found: C, 54.22; H, 4.12; N, 2.53. MS (ESI):
m/z 957 [M - CF3SO3]+.
Supporting Information Available: Crystallographic data for
6a and 6b as CIF files. This material is available free of charge via
the Internet at http://pubs.acs.org.
trans-(1-Ethyl-2,3,5-trimethylpyrazolin-4-ylidene)iodobis(triphe-
nylphosphine)palladium(II) Triflate (6b). 6b was prepared analo-
gously to 6a from 5b (62 mg, 0.15 mmol), tris(dibenzylideneac-
etone)dipalladium(0) (69 mg, 0.075 mmol), and triphenylphosphine
OM7009107
(8) Sheldrick, G. M. SHELXL-97; Universität Göttingen: Göttingen,
Germany, 1997.
1
(79 mg, 0.30 mmol). Yield: 115 mg, 0.11 mmol, 73%. H NMR