Palladium(II) pyrazolin-4-ylidenes: Substituent effects on the formation and catalytic activity of pyrazole-based remote NHC complexes

Article abstract of DOI:10.1021/om8010849

Three 4-iodopyrazolium salts with 3,5-dimethyl (3a), 3,5-diphenyl (3b), and 3,5-diisopropyl (3c) substituents, respectively, were synthesized using a modular approach. The oxidative addition of 3a-c to Pd₂(dba) ₃/PPh₃ affo

Full text of DOI:10.1021/om8010849

2778
Organometallics 2009, 28, 2778–2786
Palladium(II) Pyrazolin-4-ylidenes: Substituent Effects on
the Formation and Catalytic Activity of Pyrazole-Based Remote
NHC Complexes
Yuan Han, Li Juan Lee, and Han Vinh Huynh*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543, Singapore
ReceiVed NoVember 14, 2008
Three 4-iodopyrazolium salts with 3,5-dimethyl (3a), 3,5-diphenyl (3b), and 3,5-diisopropyl (3c)
substituents, respectively, were synthesized using a modular approach. The oxidative addition of 3a-c
to Pd2(dba)3/PPh3 afforded products (trans-4a, cis-/trans-4b and 4c) of different geometries or
connectivities, indicating a dramatic substituent effect on the formation of pyrazolin-4-ylidene complexes.
In addition, the reactions of 3a-c with Pd2(dba)3 in the presence of pyridine yielded new mixed pyrazolin-
4-ylidene/pyridine complexes (5a-c). All complexes have been fully characterized by multinuclear NMR
spectroscopies, ESI mass spectrometry, and X-ray diffraction analyses. Furthermore, an initial catalytic
study on Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions also reveals a significant
substituent effect on catalytic activities.
Introduction
Recent studies pioneered by Raubenheimer and co-workers
have shown that complexes containing N-heterocyclic carbene
ligands with remote heteroatoms (rNHC) exhibit better catalytic
performance compared to their classical NHC analogues in
certain C-C coupling reactions.1 The superiority of the former
may stem from the stronger σ-donor ability of rNHC ligands
as supported by computational studies.1a,2 Despite these promis-
ing properties, rNHCs have not yet attracted the same degree
of attention as common NHCs. In contrast to the versatile
structures known for normal NHCs,3 the structure of rNHCs
was limited to one-N-six-membered ring motifs1,2,4 derived from
pyridine (A), quinoline (B), and acridine (C) (Figure 1).
Cyclopropenylidenes (D)5 are not NHCs, but also contain a
remote heteroatom with respect to the carbene center. Only
recently, we introduced two-N-five-membered rNHCs derived
from pyrazole (E)6 as direct isomers to standard NHCs to this
little explored field.
Figure 1. Different types of rNHCs: pyridin-4-ylidene (A);
quinolin-4-ylidene (B); acridin-9-ylidene (C); cyclopropenylidene
(D); pyrazolin-4-ylidene (E).
phosphines via oxidative addition of 4-iodopyrazolium salts to
Pd2(dba)3/PPh3.6 As a continuation of our research and with the
attempt to tune the sterics and electronics around the carbene
carbon, we herein present results of our study on the influence
of 3,5-substituents in pyrazolin-4-ylidenes on complexation. In
addition, the synthesis of new mixed rNHC-pyridine complexes
is also described, extending the scope of pyrazolin-4-ylidene
ligands to phosphine-free systems. A preliminary study inves-
tigating the influence of 3,5-substituents on the catalytic
activities of the resulting complexes in Suzuki-Miyaura and
Mizoroki-Heck coupling reactions is included as well.
In our previous reports, we have established a facile synthetic
pathway to Pd(II) pyrazolin-4-ylidene complexes supported by
Results and Discussion
* Corresponding author. E-mail: chmhhv@nus.edu.sg.
Synthesis and Characterizations of 4-Iodopyrazolium
Salts. Three 4-iodopyrazolium salts (3a-c) that differ only in
their 3,5-substituents have been synthesized in three steps as
depicted in Scheme 1. This modular approach is highly
desirable, as ligands with a wide range of substitution patterns
can be conveniently obtained through the condensation of readily
available diketones with substituted hydrazines, which in turn
allows facile fine-tuning of steric and electronic properties of
the resulting rNHC ligands. The introduction of an iodo
substituent at the 4-position in the second step deactivates the
pyrazole for electrophilic attack, and thus relatively strong
alkylating reagents such as alkyl iodides and Meerwein’s salts
are required. Alkyl bromides generally afforded only negligible
yields even under harsh reaction conditions and prolonged
reaction time. When alkyl iodides are employed, the 4-iodopy-
razole has to be heated under reflux either with a large excess
(1) (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.; Roembke, P.; Julius, G. R.; Raubenheimer,
H. G.; Herrmann, W. A. AdV. Synth. Catal. 2006, 348, 1862. (c) Schneider,
S. K.; Rentzsch, C. F.; Kru¨ger, A.; Raubenheimer, H. G.; Herrmann, W. A.
J. Mol. Catal. A: Chem. 2007, 265, 50.
(2) Schneider, S. K.; Julius, G. R.; Loschen, C.; Raubenheimer, H. G.;
Frenking, G.; Herrmann, W. A. Dalton Trans. 2006, 1226.
(3) For a recent review, see: Hahn, F. E.; Jahnke, M. C. Angew. Chem.,
Int. Ed. 2008, 47, 3122.
(4) Meyer, W. H.; Deetlefs, M.; Pohlmann, M.; Scholz, R.; Esterhuysen,
M. W.; Julius, G. R.; Raubenheimer, H. G. Dalton Trans. 2004, 413.
(5) (a) Tamm, M.; Grzegorzewski, A.; Hahn, F. E. J. Organomet. Chem.
1995, 501, 309. (b) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.;
Bertrand, G. Science 2006, 312, 722. (c) Lavallo, V.; Ishida, Y.; Donnadieu,
B.; Bertrand, G. Angew. Chem., Int. Ed. 2006, 45, 6652. (d) Schoeller,
W. W.; Frey, G. D.; Bertrand, G. Chem.-Eur. J. 2008, 14, 4711. (e)
Kuchenbeiser, G.; Donnadieu, B.; Bertrand, G. J. Organomet. Chem. 2008,
693, 899. (f) Holschumacher, D.; Hrib, C. G.; Jones, P. G.; Tamm, M.
Chem. Commun. 2007, 3661.
10.1021/om8010849 CCC: $40.75
2009 American Chemical Society
Publication on Web 04/08/2009

Pyrazolin-4-ylidene Complexes of Palladium(II)
Organometallics, Vol. 28, No. 9, 2009 2779
Scheme 1. Synthetic Pathway to 4-Iodopyrazolium Salts 3a-ca
a
Only for the synthesis of 3b and 3c.
Scheme 2. Synthesis of Pyrazolin-4-ylidene-Phosphine Complexes 4a-c
or in neat alkylating agent in order to obtain a reasonable yield.6a
However, when Meerwein’s stronger trimethyloxonium salt is
used, the alkylation can proceed smoothly with a stoichiometric
amount of the reagent and under milder reaction conditions.
Furthermore, it was found that the rate of alkylation is also
affected by the 3,5-substituents. Correspondingly, the reaction
of 2b and 2c, bearing bulkier 3,5-diphenyl and 3,5-diisopropyl
substituents, respectively, required heating under reflux for an
additional day in order to obtain 3b and 3c in good yields.
The formation of all three 4-iodopyrazolium salts was
confirmed by a base peak in the positive ESI-MS spectra at
m/z ) 313 (3a), 437 (3b), and 369 (3c), respectively, corre-
sponding to the [M - BF4]+ fragment. In addition, the 13C NMR
spectra revealed C-I carbon resonances at 67.4 ppm (3a), 68.5
ppm (3b), and 60.7 ppm (3c), respectively, which are shifted
downfield upon N-methylation with respect to the analogous
carbon resonances for their neutral precursors (2a-c). Interest-
ingly, these chemical shifts decrease in the order 3b > 3a >
3c, in accordance with the electron-donating ability of the 3,5-
for the BF4 counteranion in each salt, due to the presence of
-
two NMR-active isotopes 10B and 11B.
Synthesis and Characterizations of Pd(II) Pyrazolin-4-
ylidene-Phosphine Complexes. In an attempt to obtain cationic
Pd(II) rNHC-phosphine complexes, Pd2(dba)3 was treated with
2 equiv of 3a/b/c and 4 equiv of PPh3 in dichloromethane under
reflux for 6 h according to our previously reported procedure
(Scheme 2).6b Interestingly, these three reactions gave rise to
products different in geometry or connectivity from each other,
indicating a dramatic influence of the 3,5-substituents in rNHC
ligands on complexation.
As shown in Scheme 2a, the reaction of 3a, bearing the least
bulky 3,5-dimethyl substituents, afforded the cationic complex
trans-4a in quantitative yield. The trans configuration of 4a is
indicated by its 31P NMR spectrum, which shows only one signal
at 22.9 ppm, suggesting two equivalent phosphine ligands. This
chemical shift is similar to those previously reported for other
trans-[PdI(rNHC)(PPh3)2]+ complexes based on the pyrazolin-
4-ylidene system.6b Furthermore, the 13C NMR signals for the
carbenoid carbon and the two adjacent carbon atoms C3/C5 in
complex 4a are observed at 128.3, 145.1, and 145.6 ppm,
respectively. All these signals appear as triplets due to hetero-
nuclear coupling with constants of 2J(C,P) ) 7.3 Hz and 3J(C,P)
) 3.2 Hz, respectively, again corroborating the trans arrange-
ment of the two phosphine ligands. Although the carbenoid
resonance of 128.3 ppm is more upfield than the values
commonly observed for analogous Pd(II) complexes of other
i
substituents Ph (3b) < Me (3a) < Pr (3c). Two singlets with
an intensity ratio of ∼1:4 are observed in the 19F NMR spectrum
(6) (a) Han, Y.; Huynh, H. V. Chem. Commun. 2007, 1089. (b) Han,
Y.; Huynh, H. V.; Tan, G. K. Organometallics 2007, 26, 6581. Of close
resemblance to our pyrazolin-4-ylidene ligands are five-membered-ring bent
allenes and their complexes reported by Bertrand and co-workers very
recently. Lavallo, V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2008, 47, 5411. These allenes differ from our pyrazolin-
4-ylidenes only in their N- and 3,5-substituents. However, we are in favor
of considering them as rNHCs due to the bond parameters obtained from
the molecular structures, which suggest an aromatic five-membered ring.

2780 Organometallics, Vol. 28, No. 9, 2009
Han et al.
2
constant of J(P,P) ) 24.8 Hz, pointing to two inequivalent
phosphine ligands and thus a cis geometry. In the 13C NMR
spectrum, two downfield doublets of doublets are observed at
150.5 and 148.5 ppm, corresponding to the C3/C5 carbon atoms
in the rNHC ring. In addition, the carbenoid carbon also appears
as a doublet of doublets centered at 125.0 ppm with 2J(Ptrans,C)
) 143.9 Hz and 2J(Pcis,C) ) 5.5 Hz, due to the coupling to two
inequivalent phosphorus atoms.
Upon standing in CD2Cl2, cis-4b slowly isomerizes to trans-
4b, as monitored by 31P NMR spectroscopy, which shows a
gradual decrease of the two doublets for cis-4b accompanied
by a slow increase of one singlet at 20.3 ppm for trans-4b. A
comparison of their integrals reveals a conversion of ∼9% after
28 days. However, this isomerization process is much faster in
more polar solvents such as CH3CN and DMSO. For example,
after standing in CD3CN for 2 days, cis-4b has completely
converted to trans-4b. Such a solvent dependence of the
cis-trans isomerization was also reported for a Pt(II) complex
of the type [PtCl(rNHC)(PPh3)2]+BF4-, where the rNHC is a
quinoline-4-ylidene ligand.4 The thermodynamic preference of
trans-4b over cis-4b can be explained by the transphobia effect,
a term proposed by Vicente and co-workers9 for the difficulty
of placing a phosphine ligand trans to carbon donors such as
aryl or aroyl in Pd complexes. Apparently, the isomerization
of cis-4b to trans-4b suggests that this transphobia effect is also
valid for the carbene-phosphine pair.
Figure 2. Molecular structure of the complex cation of trans-
4a · Me2CO showing 50% probability ellipsoids. BF4- counterion,
solvent molecule, and hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Pd1-C1 2.006(8),
Pd1-I1 2.6717(13), Pd1-P1 2.326(3), Pd1-P2 2.332(3), C1-C2
1.415(13), C1-C3 1.392(12), N1-C2 1.318(12), N2-C3 1.355(12),
N1-N2 1.360(11); C1-Pd1-P1 90.2(3), C1-Pd1-P2 89.0(3),
P1-Pd1-I1 90.75(7), P2-Pd1-I1 90.13(7), C2-C1-C3 103.8(8).
rNHCs4 or standard NHCs,1a,7 it is still significantly downfield-
shifted by ∆δ ) 60.9 ppm compared to the C4 carbon
resonances in its ligand precursor.
1
As expected, the H NMR spectrum of trans-4b shows a
better resolved pattern in the aromatic region compared to that
of cis-4b, which is dominated by broad signals. More impor-
tantly and different from the two doublets of doublets observed
for the C3/C5 carbon atoms in cis-4b, the analogous carbon
resonances in trans-4b arise at 149.8 and 148.6 ppm as two
triplets due to coupling with two equivalent phosphine ligands
with a constant of 3J(P,C) ) 3.7 Hz. However, the carbene signal
for trans-4b could not be clearly indentified due to the
interference of many aromatic signals in the same range of
127-133 ppm.
Single crystals were grown by slow evaporation of a
concentrated CH2Cl2/hexane solution (cis-4b) or by diffusion
of diethyl ether into a CH3CN solution (trans-4b) and subjected
to X-ray diffraction analysis. Their molecular structures are
shown in Figure 3. The Pd-Ccarbene [2.038(4) Å] and Pd-I
[2.6539(4) Å] bond lengths in cis-4b are very similar to the
corresponding parameters [Pd-Ccarbene ) 2.033(7) Å; Pd-I )
2.6501(8) Å] in trans-4b. However, a comparison of the
Pd-Ccarbene bonds in trans-4b and trans-4a [2.006(8) Å] revealed
a pronounced elongation in the former. This may be attributed
to the bulkier but less electron-donating Ph substituents in trans-
4b compared to the Me groups in trans-4a. Finally, comparison
of the Pd-P bonds in complex cis-4b revealed that the one
trans to the rNHC ligand is significantly longer than that trans
to the iodo ligand, which is consistent with a stronger trans
influence of the rNHC ligand.
Single crystals of complex 4a were obtained by slow
evaporation of a concentrated acetone/hexane solution and
analyzed by X-ray diffraction. The molecular structure is
depicted in Figure 2, and selected crystallographic data are
summarized in the Supporting Information. As found in solution,
complex 4a adopts the expected trans configuration of the
phosphine ligands in an essentially square-planar coordination
geometry. The Pd-P bonds of 2.326(3) and 2.332(3) Å and
Pd-Ccarbene bond of 2.006(8) Å fall in the range reported for
other Pd(II) pyrazolin-4-ylidene complexes.6 Furthermore, the
carbene ring plane is oriented almost perpendicular to the PdICP2
coordination plane with a dihedral angle of 89.63° to relieve
steric congestion.
In contrast to the formation of trans-4a, the reaction involving
3b, bearing the bulkier 3,5-diphenyl substituents, afforded a cis-
configured complex cis-4b as the kinetically favored product,
which slowly converts to the thermodynamically more stable
trans-isomer upon standing in solution (Scheme 2b). Complex
cis-4b was isolated in 40% yield after recrystallization from a
concentrated CH2Cl2/hexane solution. In the literature, most
complexes of the type [PdXL(PR3)2]+ (X ) halide; L ) NHC,
rNHC, or Fisher-carbene; R ) aryl or alkyl) adopt a trans
configuration with only two exceptions.1b,8 For example,
Raubenheimer and co-workers reported that the complex cis-
[PdCl(NHC)(PPh3)2]+BF4- derived from a quinolinium salt was
observed only at low temperatures of -20 °C or below, whereas
at room temperature its trans-isomer was observed exclusively.
In our case, the isomerization of cis-4b to trans-4b in dichlo-
romethane at ambient temperature is slow enough to allow for
the isolation of the former. The 31P NMR spectrum of cis-4b
shows two doublets at 28.7 and 16.3 ppm with a coupling
The oxidative addition of the bulkiest and electron-richest
salt 3c proved to be very difficult and again dramatically
different from 3a and 3b, affording a dinuclear, dicationic
complex 4c in a low yield of 8% (Scheme 2c). Attempts to
(9) (a) Vicente, J.; Abad, J. A.; Frankland, A. D.; Ram´ırez de Arellano,
M. C. Chem.-Eur. J. 1999, 5, 3066. (b) Vicente, J.; Arcas, A.; Bautista,
D.; Jones, P. G. Organometallics 1997, 16, 2127. (c) Vicente, J.; Arcas,
A.; Blasco, M. A.; Lozano, J.; Ram´ırez de Arellano, M. C. Organometallics
1998, 17, 5374. (d) Vicente, J.; Chicote, M. T.; Rubio, C.; Ram´ırez de
Arellano, M. C.; Jones, P. G. Organometallics 1999, 18, 2750. (e) Vicente,
J.; Arcas, A.; Ferna´ndez-Herna´ndez, J.; Bautista, D. Organometallics 2001,
20, 2767.
(7) Mathews, C. J.; Smith, P. J.; Welton, T.; White, A. J. P.; Williams,
D. J. Organometallics 2001, 20, 3848.
(8) Kremzow, D.; Seidel, G.; Lehmann, C. W.; Fu¨rstner, A. Chem.-
Eur. J. 2005, 11, 1833.

Pyrazolin-4-ylidene Complexes of Palladium(II)
Organometallics, Vol. 28, No. 9, 2009 2781
Figure 3. Molecular structures of the complex cations of cis-4b · CH2Cl2 (left) and trans-4b · 0.5CH3CN (right) showing 50% probability
-
ellipsoids. BF4 counterion, solvent molecules, and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg):
cis-4b · CH2Cl2: Pd1-C1 2.038(4), Pd1-I1 2.6539(4), Pd1-P1 2.3861(12), Pd1-P2 2.2882(11), C1-C2 1.402(6), C1-C3 1.397(6), N1-C2
1.350(5), N2-C3 1.361(5), N1-N2 1.370(5); C1-Pd1-P2 89.81(12), P1-Pd1-I1 86.12(3), C1-Pd1-I1 86.95(11), P1-Pd1-P2 97.16(4),
C2-C1-C3 104.4(4). trans-4b · 0.5CH3CN: Pd1-C1 2.033(7), Pd1-I1 2.6501(8), Pd1-P1 2.356(2), Pd1-P2 2.351(2), C1-C2 1.411(11),
C1-C3 1.374(11), N1-C2 1.354(11), N2-C3 1.370(10), N1-N2 1.352(11); C1-Pd1-P1 89.5(2), C1-Pd1-P2 90.6(2), P1-Pd1-I1
92.17(5), P2-Pd1-I1 87.92(5), C2-C1-C3 106.1(7).
improve the yield of 4c by varying stoichiometry and reaction
conditions were met with limited success. In all cases, the
majority of 3c remained unreacted. Despite sufficient amount
of phosphine, no mononuclear Pd(II) complex of the type
[PdI(rNHC)(PPh3)2]+BF4- was isolated from these attempts. A
CH2Cl2 solution subjected to ESI mass spectrometry confirmed
the identity of 4c by a base peak at m/z ) 738, corresponding
of the four different ligands in this complex. However, it is
expected that the Pr2-rNHC and PPh3 ligands are cis to each
i
other, in line with the transphobia effect. The cleavage of 4c
by the solvent is also supported by the positive ESI mass
spectrum measured of a CH3CN solution, which shows a base
peak at m/z ) 778 corresponding to the [PdI(NCCH3)
(iPr2-rNHC)(PPh3)]+ cation and a smaller peak at m/z ) 737
corresponding to the [PdI(iPr2-rNHC)(PPh3)]+ fragment.
X-ray diffraction analysis on single crystals of 4c obtained
by diffusion of diethyl ether into a concentrated CH3CN solution
of 4c revealed an iodo-bridged dinuclear structure (Figure 4).
The reversible decoordination of CH3CN and formation of
dimeric species under the influence of diethyl ether has been
reported by us previously.10 The molecular structure shows two
essentially square-planar Pd(II) centers each coordinated by one
phosphine and one rNHC ligand and bridged by two iodo
ligands. The two iPr2-rNHC ligands are oriented in a trans-syn
fashion to each other. The [Pd2I2] rhombus in 4c is slightly bent
with a hinge angle of ∼153°. Furthermore, the two Pd-I bonds
trans to the rNHC ligand are notably longer than those trans to
the PPh3 ligand, confirming a stronger trans influence of the
former. Efforts to obtain single crystals of the mononuclear
complex [PdI(NCCH3)(iPr2-rNHC)(PPh3)]+BF4- in pure CH3CN
were unsuccessful.
The preferable formation of dinuclear 4c over the hypothetical
mononuclear [PdI(iPr2-rNHC)(PPh3)2]+BF4- complex could be
explained by the strong electron-donating ability as well as the
pronounced steric bulk of the 3,5-diisopropyl-2-methyl-1-
phenylpyrazolin-4-ylidene ligand.
Synthesis and Characterizations of Pd(II) Pyrazolin-4-
ylidene-Pyridine Complexes. To date, all known rNHC com-
plexes contain PPh3 as a co-ligand due to the ready availability
of low-valent [M(PPh3)4] (M ) Ni, Pd, and Pt) as metal
precursors. In an attempt to extend the scope of rNHC
complexes we studied the feasibility of incorporating other
donors such as pyridine. Hence, 2 equiv of pyrazolium precur-
1
to the [M - 2BF4]2+ fragment. However, the H, 13C, and 31P
NMR spectra of 4c in CD2Cl2 are unexpectedly complicated.
In the 1H NMR spectrum, numerous doublets of different
intensities arise in the range 0.46-1.48 ppm, which are
assignable to the isopropyl-Me groups of 3,5-substituents. In
addition, a few sets of overlapping septets and singlets are
observed in the range 3.27-3.72 ppm arising from the isopropyl-
CH protons and the N-Me groups, respectively. Correspond-
ingly, the 13C and 31P NMR spectra are also rather complicated,
indicating the presence of an isomeric mixture in solution. In
principle, four geometric isomers of 4c, namely trans-anti, trans-
syn, cis-anti, and cis-syn (trans/cis refers to the arrangement of
the PPh3 ligands; anti/syn refers to the orientation of the
unsymmetrical rNHC ligands) are feasible, which would lead
to complicated spectra.
On the other hand, the respective NMR spectra of 4c
measured in CD3CN are much simpler. In the 1H NMR
spectrum, four well-resolved doublets of equal intensity are
observed at 1.39, 1.11, 0.88, and 0.35 ppm, each integrating to
three protons and thus assigned to the isopropyl-Me groups.
The isopropyl-CH protons resonate at 3.54 and 3.22 ppm as
two septets. Correspondingly, the 13C NMR spectra show four
singlets in the range 19.6-21.0 ppm and another two singlets
at 30.8 and 31.2 ppm for the isopropyl substituents. The carbene
carbon resonates as a singlet at 106.3 ppm. Finally, the 31P NMR
spectrum shows only one singlet at 19.8 ppm for the phosphine
ligand. The simple NMR spectra suggest the presence of only
one complex in solution. We anticipate that upon dissolution
in coordinating CD3CN, 4c is cleaved to form the mononuclear
complex [PdI(NCCH3)(iPr2-rNHC)(PPh3)]+BF4- (iPr2-rNHC )
3,5-diisopropyl-2-methyl-1-phenylpyrazolin-4-ylidene). At the
moment, we are still not certain about the exact configuration
(10) Han, Y.; Hong, Y.-T.; Huynh, H. V. J. Organomet. Chem. 2008,
693, 3159.

2782 Organometallics, Vol. 28, No. 9, 2009
Han et al.
Scheme 4. Improved Synthesis of Complex 5a
Since no additional iodide source was added, the formation
of the neutral diiodo complexes 5a-c required the sacrifice of
1 equiv of salt precursor 3a-c, which explains their generally
low yields. We anticipated that the use of 4-iodopyrazolium
iodides instead of the BF4- salts would lead to an improvement.
Hence, 4-iodo-2,3,5-trimethyl-1-phenylpyrazolium iodide (3a′)
was prepared by refluxing 2a in iodomethane to test this
possibility. Indeed, by oxidative addition of 2 equiv of 3a′ to
Pd2(dba)3 in the presence of pyridine the yield of complex 5a
could be improved from initially 48% to 75% (Scheme 4).
Positive mode ESI-MS spectra of complexes 5a-c revealed
isotopic clusters centered at m/z ) 498 (5a), 622 (5b), and 553
(5c), respectively, corresponding to the [M - I]+ fragment. Their
1H NMR spectra corroborate the presence of the coordinated
pyridine with the 2,6-py-H resonance shifted downfield by ∼0.5
ppm compared to the analogous signal for free pyridine. More
importantly, the 13C NMR signal of the carbenoid carbon is
observed at 99.3 (5a), 104.5 (5b), and 95.0 (5c) ppm, respec-
tively. These chemical shifts decrease in the order 5b > 5a >
5c, which correlates to the electron-donating ability of the R
substituents Ph (5b) < Me (5a) < iPr (5c). The same trend was
also observed when comparing the chemical shifts of the CI
carbon atoms in their ligand precursors 3a-c (vide supra).
Single-crystal X-ray diffraction analysis revealed that com-
plexes 5a-c all adopt a trans arrangement around the palladium
center. Two molecules were found in the unit cell of 5c, and
one of them is depicted in Figure 5 as a representative. The
carbene ring planes of all complexes are oriented almost
perpendicularly to the PdCNI2 coordination plane with dihedral
angles ranging from 73.36ꢀ to 87.32ꢀ. On the other hand, the
pyridine ring planes in 5a and 5b deviate from the perpendicular
orientation with respect to the PdCNI2 coordination plane with
torsion angles of 66.75ꢀ for 5a and 64.34ꢀ for 5b. As an
exception, that in 5c adopts a nearly right angle (88.92ꢀ and
89.22ꢀ) to the coordination plane. The Pd-Ccarbene bonds of
5a-c fall in a narrow range of 1.981(7)-1.987(4) Å, and the
Pd-N bonds amounting to 2.121(6) (5a), 2.119(4) (5b), 2.116
(4), and 2.117(4) Å (5c), respectively, are comparable to those
reported for analogues containing normal NHC ligands.12
Figure 4. Molecular structure of complex 4c showing 50%
probability ellipsoids (upper: top view; lower: side view). BF4
-
counterion, hydrogen atoms, and all phenyl substituents are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)-C(1)
2.023(16), Pd(1)-P(1) 2.267(5), Pd(1)-I(1) 2.6534(17), Pd(1)-I(2)
2.6783(17), Pd(2)-C(17) 1.998(17), Pd(2)-P(2) 2.283(5), Pd(2)-I(2)
2.6304(17), Pd(2)-I(1) 2.6825(17), C(1)-C(2) 1.36(2), C(1)-C(3)
1.35(2), N(1)-C(2) 1.36(2), N(2)-C(3) 1.351(17), N(1)-N(2)
1.323(18), C(17)-C(18) 1.43(2), C(17)-C(19) 1.41(2), N(3)-C(18)
1.307(19), N(4)-C(19) 1.41(2), N(3)-N(4) 1.378(19); C(1)-
Pd(1)-P(1) 93.7(5), C(1)-Pd(1)-I(1) 88.3(4), P(1)-Pd(1)-I(2)
93.52(12), I(1)-Pd(1)-I(2) 84.56(5), C(17)-Pd(2)-P(2) 91.4(5),
C(17)-Pd(2)-I(2) 88.6(5), P(2)-Pd(2)-I(1) 95.08(13), I(2)-
Pd(2)-I(1) 84.92(5), C(3)-C(1)-C(2) 106.0(15), C(19)-C(17)-
C(18) 104.5(15).
sors 3a/b/c were reacted with Pd2(dba)3 and 4 equiv of pyridine
in order to obtain cationic mixed rNHC-pyridine complexes of
-
the type [PdI(rNHC)(pyridine)2]+BF4 . Surprisingly, these reac-
Catalysis. The precatalysts 4a/b and 5a/b have been tested
for their catalytic activities in Suzuki-Miyaura and Mizoroki-
Heck cross-coupling reactions. Complexes 4c and 5c are
excluded in these studies due to their generally low yields. The
results summarized in Tables 1 and 2, respectively, involve
coupling of simple substrates in order to gain some insight on
the influence of 3,5-substituents in pyrazolin-4-ylidene ligands
on the catalytic activity of their complexes. The Suzuki-Miyaura
tions afforded neutral diiodo complexes of the type
[PdI2(rNHC)(pyridine)] (5a-c) in moderate to low yields of
48%, 65%, and 4%,11 respectively (Scheme 3), and no cationic
complex could be detected. In this series, the bulkiest and
electron-richest precursor 3c gave rise to a rather sluggish
reaction, again highlighting the influence of 3,5-substituents on
complexation reactions.
Scheme 3. Synthesis of Pyrazolin-4-ylidene-Pyridine Complexes 5a-c

Pyrazolin-4-ylidene Complexes of Palladium(II)
Organometallics, Vol. 28, No. 9, 2009 2783
Table 2. Mizoroki-Heck Cross-Coupling Reactions Catalyzed by
4a/b and 5a/ba
entry catalyst
aryl halide
temp [°C] t [h] yield (%)b
1
2
3
4
5
6
7
8
4a
4b
5a
5b
4a
4b
5a
5b
4a
4b
5a
5b
4a
4b
5a
5b
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
120
120
120
120
120
120
120
120
140
140
140
140
140
140
140
140
5
5
5
100
100
92
100
82
Figure 5. Molecular structure of complex 5c · 0.5H2O showing 50%
probability ellipsoids. The other molecule, hydrogen atoms, and
solvent molecules are omitted for clarity. Selected bond lengths
(Å) and angles (deg): Pd(1)-C(1) 1.983(5), Pd(1)-N(3) 2.116(4),
Pd(1)-I(1) 2.6303(6), Pd(1)-I(2) 2.6268(6), C(1)-C(2) 1.405(7),
C(1)-C(3) 1.390(7), N(1)-C(2) 1.346(6), N(2)-C(3) 1.355(6),
N(1)-N(2) 1.358(6); C(1)-Pd(1)-I(2) 88.78(13), N(3)-Pd(1)-I(2)
89.10(12), C(1)-Pd(1)-I(1) 90.27(13), N(3)-Pd(1)-I(1) 91.86(12),
C(3)-C(1)-C(2) 104.5(4).
5
22
22
22
22
24
24
24
24
24
24
24
24
85
36
31
9
33c
10
11
12
13
14
15
16
51c
20c
16c
45c
55c
0c
Table 1. Suzuki-Miyaura Cross-Coupling Reactions Catalyzed by
4a/b and 5a/b in Aqueous Mediaa
0c
a Reaction conditions: 1 mmol of aryl halide; 1.5 mmol of tert-butyl
acrylate; 3 mL of DMF; 1.5 equiv of NaOAc; 1 mmol % of catalyst.
b Yields were determined by 1H NMR spectroscopy for an average of
two runs. c With addition of 1.5 equiv of [N(n-C4H9)4]Br.
entry catalyst
aryl halide
t [h] temp [°C] yield [%]b
complexes 5a/b than for the phosphine complexes 4a/b. A
comparison among the phosphine complexes 4a/b and the
previously reported analogues with triflate counteranion6b
revealed that 4b performs slightly better than all the other
complexes with 3,5-dimethyl substituents for all four substrates
investigated here.
1
2
3
4
5
6
7
8
4a
4b
5a
5b
4a
4b
5a
5b
4a
4b
5a
5b
4a
4b
5a
5b
4-bromobenzaldehyde 19
4-bromobenzaldehyde 19
4-bromobenzaldehyde 19
4-bromobenzaldehyde 19
4-bromoacetophenone 19
4-bromoacetophenone 19
4-bromoacetophenone 19
4-bromoacetophenone 19
4-bromoanisole
4-bromoanisole
4-bromoanisole
4-bromoanisole
RT
RT
RT
RT
RT
RT
RT
RT
80
96
97
55
100
94
96
Table 2 shows the results obtained for Mizoroki-Heck cross-
coupling reactions. The coupling of aryl halides with tert-butyl
acrylate with 1 mol % catalyst loading in DMF under aerobic
conditions was chosen as a standard test reaction in this study.
Entries 1-4 show that all four precatalysts can couple activated
4-bromobenzaldehyde in quantitative or nearly quantitative
yields at 120 °C within 5 h. However, the coupling reactions
with 4-bromoacetophenone are much slower, affording yields
ranging from 31% to 85% even after 22 h (entries 5-8).
Furthermore, in cases of the more difficult substrates, e.g.,
4-bromoanisole (entries 9-12) and 4-chlorobenzaldehyde (en-
tries 13-16), only very low to moderate yields are obtained
even at elevated temperature of 140 °C and with addition of
TBAB. Again, it was found that complex 4b, with the bulky
3,5-diphenyl substituents, shows overall the best performance
in Mizoroki-Heck cross-coupling reactions. Moreover, the
phosphine complexes 4a/b are generally more active than their
counterparts with pyridine bearing the same pyrazolin-4-yl
ligands 5a/b.
31
70
9
21
21
21
21
21
21
21
21
95c
10
11
12
13
14
15
16
80
80
80
98c
100c
100c
39c
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-chlorobenzaldehyde
4-chlorobenzaldehyde
80
80
80
80
50c
21c
52c
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 addition of 1.5 equiv of [N(n-C4H9)4]Br.
1 mmol of aryl halide; 1.2 mmol of
cross-coupling reactions were performed in water under aerobic
conditions employing 1 mol % catalyst loading to allow a direct
comparison with our previously reported Pd(II) pyrazolin-4-
ylidene complexes.6b All four complexes are able to couple
activated aryl bromides at ambient temperature, affording
moderate to good yields (entries 1-8, Table 1). As for the
deactivated substrate 4-bromoanisole, very good yields
could be achieved by increasing temperature and adding
[N(n-C4H9)4]Br (TBAB) (entries 9-12, Table 1). However,
these precatalysts are not very active toward the more difficult
substrate 4-chlorobenzaldehyde, giving rise to only low to
moderate yields (entries 13-16, Table 1) under the given
conditions. Overall, it was found that complexes with the bulkier
3,5-diphenyl substituents are generally more active in the
coupling of electron-poor aryl halides compared to their
counterparts bearing 3,5-dimethyl substituents. Notably, such
a substituent effect is more pronounced for the pyridine
Conclusion
Three 4-iodopyrazolium salts with 3,5-dimethyl (3a), 3,5-
diphenyl (3b), and 3,5-diisopropyl (3c) substituents, respectively,
were prepared in good yields in an attempt to tune the sterics
and electronics around the carbene center in the resulting
pyrazolin-4-ylidene ligands. Oxidative addition of 3a-c to
Pd2(dba)3/PPh3 afforded products (trans-4a, cis-/trans-4b and
4c) different in geometry or connectivity, highlighting the
dramatic influence of the 3,5-substituents in pyrazolin-4-ylidene

2784 Organometallics, Vol. 28, No. 9, 2009
Han et al.
21.4 (s, CH(CH3)2). Anal. Calcd for C15H19IN2: C, 50.86; H, 5.41;
N, 7.91. Found: C, 51.02; H, 5.24; N, 7.91. MS (ESI): m/z ) 355
[M + H]+.
ligands on complexation. A detailed study on isomerization of
complex 4b revealed that trans-4b is the thermodynamically
more stable isomer, in line with the transphobia effect. In
addition, a series of new mixed rNHC-pyridine complexes
derived from 3a-c were also synthesized, which again dem-
onstrates the substituent effect on complexation. The substituent
effect on catalytic activities of complexes 4a/b and 5a/b was
also observed in both Suzuki-Miyaura and Mizoroki-Heck
cross-coupling reactions. Current research in our laboratory is
ongoing to extend the scope of rNHCs to functionalized
pyrazolin-4-ylidene complexes and their applications.
4-Iodo-2,3,5-trimethyl-1-phenylpyrazolium Tetrafluoroborate
(3a). 2a (597 mg, 2.0 mmol) was dissolved in dry CH2Cl2 (10 mL),
and the resulting solution was transferred to a suspension of
trimethyloxonium tetrafluoroborate (355 mg, 2.4 mmol) in dry
CH2Cl2 (10 mL) via cannula. The reaction mixture was stirred at
ambient temperature overnight under nitrogen. All volatiles were
removed under reduced pressure, and the off-white solid was
washed with diethyl ether (3 × 20 mL). The residue was dried in
vacuo to give the product as a white powder. Yield: 761 mg, 1.9
1
Experimental Section
mmol, 95%. H NMR (300 MHz, CDCl3): δ 7.61-7.75 (m, 5 H,
Ar-H), 3.79 (s, 3 H, NCH3), 2.63 (s, 3 H, CH3), 2.29 (s, 3 H, CH3).
13C(1H) NMR (75.48 MHz, CDCl3): 150.8, 149.4 (s, CCH3), 133.6,
132.0, 131.7, 129.4 (s, Ar-C), 67.4 (s, CI), 36.8 (s, NCH3), 14.7,
14.4 (s, CCH3). 19F(1H) NMR (282 MHz, CDCl3): -77.94, -77.89
(s, 4 F, BF4). Anal. Calcd for C12H14BF4IN2: C, 36.04; H, 3.53; N,
7.00. Found: C, 35.84; H, 3.50; N, 6.92. MS (ESI): m/z 313
[M - BF4]+.
4-Iodo-2,3,5-trimethyl-1-phenylpyrazolium Iodide (3a′). 2a (1.475
g, 4.95 mmol) was dissolved in iodomethane (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.
Yield: 1.097 g, 2.49 mmol, 50%. 1H NMR (500 MHz, d6-DMSO):
δ 7.81-7.70 (m, 5 H, Ar-H), 3.69 (s, 3 H, NCH3), 2.56 (s, 3 H,
CH3), 2.21 (s, 3 H, CH3). 13C(1H) NMR (125.76 MHz, d6-DMSO):
148.8, 148.2 (s, CCH3), 132.5, 131.4, 130.5, 128.7 (s, Ar-C), 70.2
(s, CI), 36.0 (s, NCH3), 13.7, 13.4 (s, CCH3). Anal. Calcd for
C12H14I2N2: C, 32.75; H, 3.21; N, 6.37. Found: C, 32.71; H, 3.25;
N, 6.25. MS (ESI): m/z 313 [M - I]+.
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. CH2Cl2 was dried with
CaH2 and distilled under nitrogen using standard Schlenk tech-
niques. Tris(dibenzylideneacetone)dipalladium(0) was received from
Alfa Aesar. 3,5-Dimethyl-1-phenylpyrazole (1a) and 4-iodo-3,5-
dimethyl-1-phenylpyrazole (2a) were synthesized following reported
procedures.6b 1,3,5-Triphenylpyrazole (1b) and 3,5-diisopropyl-1-
phenylpyrazole (1c) were prepared in analogy with 1a. 4-Iodo-
1,3,5-triphenylpyrazole (2b) and 4-iodo-3,5-diisopropyl-1-phe-
nylpyrazole (2c) were synthesized in analogy with 2a. 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, 13C) or externally to 85% H3PO4 (31P) and CF3CO2H (19F).
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.
1,3,5-Triphenylpyrazole (1b). 1b was synthesized from diben-
zoylmethane (2.242 g, 10.0 mmol) and phenylhydrazinium chloride
(1446 mg, 10.0 mmol). Yield: 2.542 g, 8.58 mmol, 86%. The purity
of 1b was verified by spectroscopic comparison with an authentic
sample from Sigma-Aldrich.
4-Iodo-2-methyl-1,3,5-triphenylpyrazolium Tetrafluoroborate
(3b). 2b (718 mg, 1.7 mmol) was dissolved in dry CH2Cl2 (10 mL),
and the resulting solution was transferred to a suspension of
trimethyloxonium tetrafluoroborate (281 mg, 1.9 mmol) in dry
CH2Cl2 (10 mL) via cannula. The reaction mixture was stirred at
ambient temperature for a day and then under reflux for another
day under an inert nitrogen atmosphere. After cooling to ambient
temperature, all volatiles were removed in vacuo. The residue was
washed with diethyl ether (3 × 20 mL) and dried under vacuum to
afford the product as a white powder. Yield: 873 mg, 1.66 mmol,
98%. 1H NMR (300 MHz, CDCl3): δ 7.79-7.24 (m, 15 H, Ar-H),
3.72 (s, 3 H, NCH3). 13C(1H) NMR (75.48 MHz, CDCl3): 152.6,
151.9 (s, CN), 132.9, 132.8, 132.1, 131.5, 131.0, 130.9, 130.1,
130.0, 129.3, 127.0, 126.9 (s, Ar-C), 68.5 (s, CI), 38.0 (s, NCH3).
19F(1H) NMR (282 MHz, CDCl3): -77.09, -77.14 (s, 4 F, BF4).
Anal. Calcd for C22H18BF4IN2: C, 50.42; H, 3.46; N, 5.35. Found:
C, 50.41; H, 3.53; N, 5.34. MS (ESI): m/z 437 [M - BF4]+.
3,5-Diisopropyl-1-phenylpyrazole (1c). 1c was synthesized from
2,2,6,6-tetramethyl-3,5-heptanedione (1.716 mL, 10.0 mmol) and
phenylhydrazinium chloride (1.446 g, 10.0 mmol). Yield: 1.864 g,
1
8.16 mmol, 82%. H NMR (300 MHz, CDCl3): δ 7.47-7.33 (m,
3
5 H, Ar-H), 6.03 (s, 1 H, CH), 3.01 (m, J(H,H) ) 6.9 Hz, 2 H,
3
CH(CH3)2), 1.30 (d, J(H,H) ) 6.9 Hz, 6 H, CH(CH3)2), 1.17 (d,
3J(H,H) ) 6.9 Hz, 6 H, CH(CH3)2). 13C(1H) NMR (75.47 MHz,
CDCl3): 159.9, 151.5 (s, CN), 140.8, 129.7, 128.4, 126.7 (s, Ar-
C), 100.2 (s, CH), 28.6, 26.2 (s, CH(CH3)2), 23.7, 23.6 (s,
CH(CH3)2). Anal. Calcd for C15H20N2: C, 78.90; H, 8.83; N, 12.27.
Found: C, 78.10; H, 8.39; N, 12.05. MS (ESI): m/z 229 [M + H]+.
4-Iodo-1,3,5-triphenylpyrazole (2b). 2b was synthesized from
1b (2.345 g, 7.91 mmol). Slow evaporation of a diethyl ether
solution of the crude product afforded the pure product as colorless
1
crystals. Yield: 1.455 g, 3.45 mmol, 43%. H NMR (300 MHz,
CDCl3): δ 8.02-7.30 (m, 15 H, Ar-H). 13C(1H) NMR (75.47 MHz,
CDCl3): 153.7, 146.1 (s, CN), 140.6, 133.5, 131.2, 130.9, 129.7,
129.5, 129.3, 129.2, 129.1, 128.9, 128.2, 125.5 (s, Ar-C), 64.3 (s,
CI). Anal. Calcd for C21H15IN2: C, 59.73; H, 3.58; N, 6.63. Found:
C, 59.56; H, 3.63; N, 6.65. MS (ESI): m/z 423 [M + H]+.
4-Iodo-3,5-diisopropyl-1-phenylpyrazole (2c). 2c was synthe-
sized. from 1c (1.864 g, 8.16 mmol). Yield: 2.553 g, 7.21 mmol,
4-Iodo-3,5-diisopropyl-2-methyl-1-phenylpyrazolium Tetrafluo-
roborate (3c). 3c was prepared from 2c (1.806 g, 5.1 mmol) in
analogy with 3b except that the residue was washed with ethyl
acetate (4 × 20 mL) instead of diethyl ether (3 × 20 mL). Yield:
1.768 g, 3.88 mmol, 76%. 1H NMR (300 MHz, CDCl3): δ
7.71-7.61 (m, 5 H, Ar-H), 3.70 (s, 3 H, NCH3), 3.44 (m, 3J(H,H)
3
1
) 7.1 Hz, 1 H, CH(CH3)2), 2.96 (m, J(H,H) ) 7.1 Hz, 1 H,
88%. H NMR (300 MHz, CDCl3): δ 7.49-7.31 (m, 5 H, Ar-H),
3
3
3
CH(CH3)2), 1.51 (d, J(H,H) ) 7.1 Hz, 6 H, CH(CH3)2), 1.25 (d,
3.10 (m, J(H,H) ) 6.9 Hz, 1 H, CH(CH3)2), 3.04 (m, J(H,H) )
6.9 Hz, 1 H, CH(CH3)2), 1.32 (d, 3J(H,H) ) 6.9 Hz, 6 H,
3J(H,H) ) 7.1 Hz, 6 H, CH(CH3)2). 13C(1H) NMR (75.47 MHz,
CDCl3): 155.8, 155.3 (s, CN), 133.7, 132.0, 131.5, 130.1 (s, Ar-
C), 60.7 (s, CI), 36.4 (s, NCH3), 29.2, 29.0 (s, CH(CH3)2), 20.0,
19.7 (s, CH(CH3)2). 19F(1H) NMR (282 MHz, CDCl3): -77.32,
CH(CH3)2), 1.31 (d, J(H,H) ) 6.9 Hz, 6 H, CH(CH3)2). 13C(1H)
NMR (75.47 MHz, CDCl3): 159.6, 148.0 (s, CN), 140.9, 129.8,
3
129.2, 127.4 (s, Ar-C), 59.2 (s, CI), 28.7, 27.3 (s, CH(CH3)2), 22.5,

Pyrazolin-4-ylidene Complexes of Palladium(II)
Organometallics, Vol. 28, No. 9, 2009 2785
-77.38 (s, 4 F, BF4). Anal. Calcd for C16H22BF4IN2: C, 42.14; H,
4.86; N, 6.14. Found: C, 41.96; H, 4.79; N, 6.08. MS (ESI): m/z )
369 [M - BF4]+.
3.7 Hz, CN), 135.5, 133.1, 132.3, 132.0, 131.5, 131.4, 131.2, 131.1,
131.0, 130.5, 130.3, 130.0, 129.9, 128.8, 128.7, 128.6, 128.5, 128.4,
128.3, 128.2, 1281.1, 128.0, 127.8 (Ar-C and Ccarbene, coupling to
phosphorus is not considered), 36.8 (s, NCH3). 31P(1H) NMR
(202.45 MHz, d6-DMSO): 20.6 (s, 2 P, PPh3). 19F(1H) NMR (282
MHz, d6-DMSO): -72.30, -72.35 (s, 4 F, BF4). Anal. Calcd for
C58H48BF4IN2P2Pd: C, 60.31; H, 4.19; N, 2.43. Found: C, 60.25;
H, 4.35; N, 2.39. MS (ESI): m/z 1067 [M - BF4]+.
trans-Iodo-(2,3,5-trimethyl-1-phenylpyrazolin-4-ylidene)bis(tri-
phenylphosphine)palladium(II) Tetrafluoroborate (4a). Tris(diben-
zylideneacetone)dipalladium(0) (92 mg, 0.1 mmol) and triphe-
nylphosphine (105 mg, 0.4 mmol) were dissolved in dry CH2Cl2
(20 mL) and stirred at ambient temperature for 10 min under
nitrogen. To the resulting dark red solution was added a solution
of 3a (80 mg, 0.2 mmol) in dry CH2Cl2 (10 mL) via cannula. The
reaction mixture was heated under reflux for 6 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 Na2SO4,
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 (4 × 30
mL) to give the product as a light yellow powder. Yield: 204 mg,
Di-µ-iodobis(3,5-diisopropyl-2-methyl-1-phenylpyrazolin-4-
ylidene)bis(triphenylphosphine)dipalladium(II) Ditetrafluorobo-
rate (4c). Tris(dibenzylideneacetone)dipalladium(0) (137 mg, 0.15
mmol) and triphenylphosphine (79 mg, 0.3 mmol) were dissolved
in dry CH2Cl2 (20 mL) and stirred at ambient temperature for 10
min under nitrogen. To the resulting dark red solution was added
a solution of 3c (137 mg, 0.3 mmol) in dry CH2Cl2 (15 mL) via
cannula. The reaction mixture was heated under reflux for 6 h in
an inert nitrogen atmosphere and then cooled to ambient temper-
ature. The resulting mixture was filtered through Celite, and the
solvent of the filtrate was removed in vacuo. The residue was
washed with diethyl ether (4 × 30 mL) and then recrystallized by
slow evaporation of a concentrated methanol solution. The resultant
crystals were collected by filtration and washed with ice-cold
methanol. The remaining crystals were recrystallized twice by
diffusing diethyl ether into a concentrated acetonitrile solution to
afford the analytically pure product. Yield: 20 mg, 0.012 mmol,
8%. 1H NMR (500 MHz, CD2Cl2): δ 7.71-7.01 (m, 40 H, Ar-H),
3.72-3.63 (m, 2 H, CH(CH3)2), 3.45-3.41 (m, 6 H, NCH3),
3.38-3.26 (m, 2 H, CH(CH3)2), 1.48-1.38 (m, 6 H, CH(CH3)2),
1.19-1.11 (m, 6 H, CH(CH3)2), 1.05-0.97 (m, 6 H, CH(CH3)2),
0.53-0.46 (m, 6 H, CH(CH3)2). 31P(1H) NMR (202.45 MHz,
CD2Cl2): 27.4, 27.3, 24.0, 23.9 (s, PPh3). 19F(1H) NMR (282 MHz,
CD2Cl2): -77.45, -77.50 (s, 4 F, BF4). 13C(1H) NMR (125.77 MHz,
CD2Cl2): 155.2-154.8 (m, CN), 135.0, 133.1, 133.0, 132.3, 132.2,
131.05, 131.01, 130.99, 130.8, 129.7, 129.50, 129.47, 129.27,
1
0.2 mmol, 99%. H NMR (500 MHz, CD2Cl2): δ 7.63-7.44 (m,
33 H, Ar-H), 6.77-6.76 (m, 2 H, Ar-H), 3.07 (s, 3 H, NCH3),
2.00 (s, 3 H, CH3), 1.73 (s, 3 H, CH3). 31P(1H) NMR (202.45 MHz,
CD2Cl2): 22.9 (s, 2 P, PPh3). 19F(1H) NMR (282 MHz, CD2Cl2):
-77.44, -77.38 (s, 4 F, BF4). 13C(1H) NMR (125.77 MHz, CD2Cl2):
145.6 (t, 3J(P,C) ) 3.2 Hz, CN), 145.1 (t, 3J(P,C) ) 3.2 Hz, CN),
135.0 (t, 2/3J(P,C) ) 6.0 Hz, Ar-C), 132.3 (s, Ar-C), 131.7 (t, 1J(P,C)
) 24.7 Hz, Ar-C), 131.5, 131.1, 130.8 (s, Ar-C), 128.6 (t, 2/3J(P,C))
2
5.0 Hz, Ar-C), 128.3 (t, J(P,C) ) 7.3 Hz, Ccarbene), 127.7 (s, Ar-
C), 34.9 (s, NCH3), 14.8, 14.8 (s, CH3). Anal. Calcd for
C48H44BF4IN2P2Pd: C, 55.92; H, 4.30; N, 2.72. Found: C, 57.03;
H, 4.61; N, 2.97 (better values could not be obtained with
spectroscopically pure samples). MS (ESI): m/z 943 [M - BF4]+.
cis-Iodo-(2-methyl-1,3,5-triphenylpyrazolin-4-ylidene)bis(triphe-
nylphosphine)palladium(II) Tetrafluoroborate (cis-4b). Tris(diben-
zylideneacetone)dipalladium(0) (92 mg, 0.1 mmol) and triphe-
nylphosphine (105 mg, 0.4 mmol) were dissolved in dry CH2Cl2
(20 mL) and stirred at ambient temperature for 10 min under
nitrogen. To the resulting dark red solution was added a solution
of 3b (105 mg, 0.2 mmol) in dry CH2Cl2 (10 mL) via cannula.
The reaction mixture was heated under reflux for 6 h in an inert
nitrogen atmosphere and then cooled to ambient temperature. The
resulting mixture was filtered through Celite, and the solvent of
the filtrate was removed in vacuo. The residue was washed with
diethyl ether (4 × 30 mL) and then recrystallized by slow
evaporation of a concentrated CH2Cl2/hexane solution. The resultant
yellow crystals were collected by filtration and washed with small
portions of acetone to afford the analytically pure product. Yield:
95 mg, 0.08 mmol, 40%. 1H NMR (500 MHz, CD2Cl2): δ
8.24-6.94 (m, 45 H, Ar-H), 3.50 (s, 3 H, NCH3). 13C(1H) NMR
(125.76 MHz, CD2Cl2): 150.5, 148.5 (dd, 3J(P,C) ) 7.3 Hz, 3J(P,C)
) 3.7 Hz, CN), 135.5, 134.0, 133.1, 132.3, 132.0, 131.9, 131.8,
131.7, 131.2, 130.8, 130.5, 130.3, 130.1, 130.0, 129.0, 128.8, 128.6,
128.5, 128.4, 128.3, 128.2, 128.1, 128.0 (Ar-C, coupling to
2
129.23, 129.19, 129.15, 129.09, 129.06 (Ar-C), 118.8 (d, J(P,C)
) 8.2 Hz, Ccarbene), 35.5, 35.4 (s, NCH3), 30.81, 30.80, 30.78, 30.49,
30.46, 30.41 (s, CH(CH3)2), 21.7, 21.60, 21.57, 20.68, 20.66, 20.61,
20.56, 20.55, 20.06, 20.00, 19.94, 19.92 (s, CH(CH3)2). Anal. Calcd
for C68H74B2F8I2N4P2Pd2: C, 49.51; H, 4.52; N, 3.40. Found: C,
49.17; H, 4.70; N, 3.33. MS (ESI): m/z 738 [M - 2BF4]2+
.
trans-Diiodo-(2,3,5-trimethyl-1-phenylpyrazolin-4-ylidene)(pyr-
idine)palladium(II) (5a). Method A: Tris(dibenzylideneacetone)-
dipalladium(0) (247 mg, 0.27 mmol) and pyridine (88 µL, 1.09
mmol) were dissolved in dry CH2Cl2 (20 mL) and stirred at ambient
temperature for 10 min under nitrogen. To the resulting dark red
solution was added a solution of 3a (216 mg, 0.54 mmol) in dry
CH2Cl2 (10 mL) via cannula. The reaction mixture was heated under
reflux for 6 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 Na2SO4, and the solvent was removed
under vacuum. The residue was washed with hexane (4 × 30 mL)
and then recrystallized by slow evaporation of a concentrated
acetone solution to afford the product as orange crystals. Yield: 84
mg, 0.13 mmol, 48%. Method B: The procedure is similar to that
of method A except that in method B tris(dibenzylideneacetone)-
dipalladium(0) (183 mg, 0.2 mmol), pyridine (32 µL, 0.4 mmol),
and 3a′ (176 mg, 0.4 mmol) were used. Yield: 189 mg, 0.3 mmol,
75%. 1H NMR (500 MHz, CD2Cl2): δ 9.06-9.05 (m, 2 H, Ar-H),
7.73-7.69 (m, 1 H, Ar-H), 7.65-7.61 (m, 3 H, Ar-H), 7.33-7.28
(m, 4 H, Ar-H), 3.47 (s, 3 H, NCH3), 2.66 (s, 3 H, CH3), 2.37 (s,
3 H, CH3). 13C(1H) NMR (125.76 MHz, CD2Cl2): 153.9 (s, Ar-C),
149.3, 148.7 (s, CCH3), 137.3, 133.4, 131.8, 130.7, 128.7, 124.2
2
phosphorus is not considered), 125.0 (dd, J(P,C)trans ) 143.9 Hz,
2J(P,C)cis ) 5.5 Hz, Ccarbene), 37.5 (s, NCH3). 31P(1H) NMR (202.45
2
MHz, CD2Cl2): 28.7 (d, J(P,P) ) 24.8 Hz, 1 P, PPh3), 16.3 (d,
2J(P,P) ) 24.8 Hz, 1 P, PPh3). 19F(1H) NMR (282 MHz, CD2Cl2):
-77.65, -77.70 (s, 4 F, BF4). Anal. Calcd for C58H48BF4IN2P2Pd:
C, 60.31; H, 4.19; N, 2.43. Found: C, 60.04; H, 4.22; N, 2.49. MS
(ESI): m/z 1067 [M - BF4]+.
trans-Iodo-(2-methyl-1,3,5-triphenylpyrazolin-4-ylidene)bis(tri-
phenylphosphine)palladium(II) Tetrafluoroborate (trans-4b). cis-
4b (58 mg, 0.05 mmol) was dissolved in acetonitrile (5 mL) and
kept for 3 days. Upon evaporation of acetonitrile at ambient
temperature, the product was obtained as yellow crystals in a
1
quantitative yield. H NMR (500 MHz, d6-DMSO): δ 7.71-6.52
(m, 45 H, Ar-H), 3.50 (s, 3 H, NCH3). 13C(1H) NMR (125.76 MHz,
3
3
d6-DMSO): 149.8 (t, J(P,C) ) 3.7 Hz, CN), 148.6 (t, J(P,C) )

2786 Organometallics, Vol. 28, No. 9, 2009
Han et al.
(s, Ar-C), 99.3 (s, Ccarbene), 34.4 (s, NCH3), 16.9, 16.8 (s, CCH3).
Anal. Calcd for C17H19I2N3Pd: C, 32.64; H, 3.06; N, 6.72. Found:
C, 32.20; H, 3.04; N, 6.36. MS (ESI): m/z 498 [M - I]+.
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 10 mL amount 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-Diiodo-(2-methyl-1,3,5-triphenylpyrazolin-4-ylidene)(pyr-
idine)palladium(II) (5b). Tris(dibenzylideneacetone)dipalladium(0)
(183 mg, 0.2 mmol) and pyridine (65 µL, 0.8 mmol) were dissolved
in dry CH2Cl2 (20 mL) and stirred at ambient temperature for 10
min under nitrogen. To the resulting dark red solution was added
a solution of 3b (210 mg, 0.4 mmol) in dry CH2Cl2 (10 mL) via
cannula. The reaction mixture was heated under reflux for 6 h in
an inert nitrogen atmosphere and then cooled to ambient temper-
ature. The resulting mixture was filtered through Celite, and the
solvent of the filtrate was removed under vacuum. The residue was
washed with hexane (4 × 30 mL) and then recrystallized by slow
evaporation of a concentrated CH2Cl2 solution. The resultant orange
crystals were collected and dissolved in a minimal amount of
CH2Cl2. Passing the CH2Cl2 solution through a short plug of silica
gel followed by removal of the solvent in vacuo afforded the product
as an orange solid. Yield: 95 mg, 0.13 mmol, 65%. 1H NMR (500
MHz, CD2Cl2): δ 8.79-8.77 (m, 2 H, Ar-H), 8.20-8.19 (m, 2 H,
Ar-H), 8.03 -8.00 (m, 2 H, Ar-H), 7.66-7.33 (m, 12 H, Ar-H),
7.20-7.17 (m, 2 H, Ar-H), 3.59 (s, 3 H, NCH3). 13C(1H) NMR
(125.76 MHz, CD2Cl2): 153.8 (s, Ar-C), 152.4, 151.1 (s, CN), 137.2,
134.5, 131.9, 131.7, 131.6, 131.2, 130.5, 130.1, 129.4, 129.3, 128.6,
128.0, 124.2 (s, Ar-C), 104.5 (s, Ccarbene), 36.4 (s, NCH3). Anal.
Calcd for C27H23I2N3Pd: C, 43.25; H, 3.09; N, 5.60. Found: C,
43.24; H, 3.18; N, 5.53. MS (ESI): m/z 622 [M - I]+.
1
analyzed by H NMR spectroscopy.
General Procedure for the Mizoroki-Heck Cross-Coupling
Reaction. In a typical run, a test tube was charged with a mixture
of aryl halide (1.0 mmol), tert-butyl acrylate (1.5 mmol), anhy-
drous sodium acetate (1.5 mmol), catalyst (0.01 mmol), and
[N(n-C4H9)4]Br (1.5 mmol) (for entries 9-16 in Table 2). To the
mixture was then added DMF (3 mL). The reaction mixture was
vigorously stirred at the appropriate temperature. After the desired
reaction time, the solution was allowed to cool. A 10 mL amount
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
1
product, which was analyzed by H NMR spectroscopy.
X-ray Diffraction Studies. X-ray data were collected with a
Bruker AXS SMART APEX diffractometer, using Mo KR radiation
at 295 K (4a) or 223 K (4b/c and 5a-c), with the SMART suite
of programs.13 Data were processed and corrected for Lorentz and
polarization effects with SAINT14 and for absorption effect with
SADABS.15 Structural solution and refinement were carried out
with the SHELXTL suite of programs.16 The structure was solved
by direct methods to locate the heavy atoms, followed by difference
maps for the light, non-hydrogen atoms. All hydrogen atoms were
put at calculated positions. All non-hydrogen atoms were generally
given anisotropic displacement parameters in the final model. In
the structure of 4c, one methyl group of each isopropyl substituent
is disordered into two positions with occupancy ratio 46:54 and
trans-Diiodo-(3,5-diisopropyl-2-methyl-1-phenylpyrazolin-4-
ylidene)(pyridine)palladium(II) (5c). Tris(dibenzylideneacetone)-
dipalladium(0) (183 mg, 0.2 mmol) and pyridine (65 µL, 0.8 mmol)
were dissolved in dry CH2Cl2 (20 mL) and stirred at ambient
temperature for 10 min under nitrogen. To the resulting dark red
solution was added a solution of 3c (182 mg, 0.4 mmol) in dry
CH2Cl2 (10 mL) via cannula. The reaction mixture was heated under
reflux for 6 h in an inert nitrogen atmosphere and then cooled to
ambient temperature. The resulting mixture was filtered through
Celite, and the solvent of the filtrate was removed under vacuum.
The residue was washed with hexane (4 × 30 mL) and then
subjected to column chromatography (SiO2, diethyl ether). The third
band (Rf ) 0.5) was collected and dried in vacuo to give an or-
ange solid, which was recrystallized by diffusing pentane into a
concentrated CH2Cl2 solution to afford the product as orange
crystals. Yield: 6 mg, 0.01 mmol, 4%. 1H NMR (500 MHz,
CD2Cl2): δ 9.07-9.06 (m, 2 H, Ar-H), 7.72-7.60 (m, 4 H, Ar-H),
7.38-7.37 (m, 2 H, Ar-H), 7.31-7.28 (m, 2 H, Ar-H), 3.91 (m,
3J(H,H) ) 6.9 Hz, 1 H, CH(CH3)2), 3.44 (s, 3 H, NCH3), 3.44 (m,
-
76:34, respectively, and the fluorine atoms of one BF4 are also
disordered into two positions with occupancy 55:45. A summary
of the most important crystallographic data of 4a-c and 5a-c is
given in the Supporting Information.
Acknowledgment. We thank the National University of
Singapore for financial support (Grant No. R 143-000-327-
133) and the CMMAC staff of our department for technical
assistance.
Supporting Information Available: Crystallographic data and
CIF files for complexes 4a-c and 5a-c and ORTEP plots with
selected bond lengths and angles for complexes 5a/b. This material
is available free of charge via the Internet at http://pubs.acs.org.
3
3J(H,H) ) 6.9 Hz, 1 H, CH(CH3)2), 1.70 (d, J(H,H) ) 6.9 Hz, 6
OM8010849
H, CH(CH3)2), 1.45 (d, 3J(H,H) ) 6.9 Hz, 6 H, CH(CH3)2). 13C(1H)
NMR (125.76 MHz, CD2Cl2): 155.5, 155.3 (s, CN), 154.1 (s, Ar-
C), 137.2, 133.8, 132.1, 130.5, 129.8, 124.3 (s, Ar-C), 95.0 (s,
Ccarbene), 34.5 (s, NCH3), 30.2, 29.8 (s, CH(CH3)2), 22.2, 21.0 (s,
CH(CH3)2). Elemental analysis could not be obtained due to the
low yield of the product. MS (ESI): m/z 553 [M - I]+.
(11) Yields are calculated on the basis of the pyrazolium precursors
3a/b/c, which are the limiting reagents in these reactions.
(12) (a) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.; Hadei, N.; Chass,
G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem.-Eur. J. 2006,
12, 4743. (b) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Aldrichim.
Acta 2006, 39, 97. (c) Han, Y.; Huynh, H. V.; Tan, G. K. Organometallics
2007, 26, 6447.
(13) SMART Version 5.628; Bruker AXS Inc.: Madison, WI, 2001.
(14) SAINT+ Version 6.22a; Bruker AXS Inc.: Madison, WI, 2001.
(15) Sheldrick, G. W. SADABS Version 2.10; University of Go¨ttingen,
2001.
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 1). To the mixture was added
(16) SHELXTL Version 6.14; Bruker AXS Inc.: Madison, WI, 2000.