Correlation of spectroscopically determined ligand donor strength and nucleophilicity of substituted pyrazoles

Article abstract of DOI:10.1039/c2dt30526g

The relative ligand donor strengths of 10 pyrazole-derived ligands has been determined with great accuracy, making use of the interdependence between the donor strength of the co-ligand and the ¹³C NMR chemical shift of the ⁱPr₂-bimy carbene signal in trans-[PdBr₂( ⁱPr₂-bimy)L] complexes (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = pyrazole-derived ligand). Even subtle variations in the substitution pattern of the pyrazole backbone up to three bonds away from the coordinating nitrogen could be detected reliably using this methodology. Alkylation experiments conducted on the pyrazoles using electrophiles of varied reactivity (ethyl bromide, ethyl iodide, and trimethyloxonium tetrafluoroborate) served as a benchmark to rank the pyrazoles in three groups of gradually increasing nucleophilicity, which correlated well with their determined donor strength.

Full text of DOI:10.1039/c2dt30526g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2012, 41, 8600
www.rsc.org/dalton
PAPER
Correlation of spectroscopically determined ligand donor strength and
nucleophilicity of substituted pyrazoles†
Jan Christopher Bernhammer and Han Vinh Huynh*
Received 5th March 2012, Accepted 4th May 2012
DOI: 10.1039/c2dt30526g
The relative ligand donor strengths of 10 pyrazole-derived ligands has been determined with great
accuracy, making use of the interdependence between the donor strength of the co-ligand and the
13C NMR chemical shift of the iPr2-bimy carbene signal in trans-[PdBr2(iPr2-bimy)L] complexes
(iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = pyrazole-derived ligand). Even subtle
variations in the substitution pattern of the pyrazole backbone up to three bonds away from the
coordinating nitrogen could be detected reliably using this methodology. Alkylation experiments
conducted on the pyrazoles using electrophiles of varied reactivity (ethyl bromide, ethyl iodide, and
trimethyloxonium tetrafluoroborate) served as a benchmark to rank the pyrazoles in three groups of
gradually increasing nucleophilicity, which correlated well with their determined donor strength.
An alternative electronic parameter, introduced by Lever,
Introduction
correlates the E0 value of a redox couple (most commonly
RuII/RuIII) of complexes which incorporate the ligands of interest
The catalytic properties of transition metal complexes can be
with the donor abilities of these ligands.4 Lever’s electronic para-
fine-tuned by carefully choosing ligands of appropriate steric
demand and donor strength. In order to facilitate rational catalyst
design, on-going efforts have been made to provide a reliable
scale to compare the donor abilities of Werner-type and organo-
metallic ligands. This development is still in progress, and up to
now, several methodologies have emerged.1
meter (LEP) is available for a wide range of Werner-type
ligands. Comparison between these two scales is difficult, since
only for a handful of ligands both the TEP and the LEP values
have been determined.5
More recently, we have proposed a versatile new electronic
parameter based on the chemical shift of the carbene carbon in
complexes of the type trans-[PdBr2(iPr2-bimy)L], which allows
easy determination of both Werner-type and organometallic
ligands.6 A higher donor strength of the transoid ligand L leads
to a reduction in the Lewis acidity of the metal centre, which
in turn leads to a downfield shift of the carbene carbon in the
iPr2-bimy ligand. Such an effect has been observed before,7 and
allows a direct assessment of the donor strength by a simple
13C NMR spectroscopic measurement. In contrast to IR spectro-
scopy or electrochemical measurements, 13C NMR spectra
exhibit very sharp resonances, allowing for a highly improved
accuracy in the determination of the electronic parameter as com-
pared to TEP and LEP. Moreover, the required complex probes
can be easily prepared by reacting the ligand of interest with the
readily available dimeric complex [PdBr2(iPr2-bimy)]2 in high
yields without any toxic and hazardous chemicals involved.8
The determination of the electronic parameter can be achieved
via a simple, non-destructive and routine measurement.
The most commonly used experimental procedure, introduced
by Tolman, relies on the determination of the CO A1 band in
IR spectra of [Ni(CO)3L] complexes, which in turn is directly
influenced by the amount of Ni–CO backdonation induced by
the electron donation of the ligand L.2 While Tolman’s electronic
parameter (TEP) is indicative of the donating ability of a wide
range of phosphines, which are among the most widely used
ligands in transition metal catalysis, most Werner-type and other
ligands of interest cannot be easily characterized that way. Fur-
thermore, the synthesis of [Ni(CO)3L] complexes requires the
handling of highly toxic [Ni(CO)4], making the determination of
donor strength by this route potentially hazardous. To address
this concern, the TEP concept has been recently extended and
correlated to the Rh(I) and Ir(I) complexes [MX(CO)2L] (M =
Rh, Ir; X = halide).3 Nevertheless, ligands of interest that
compete with the COs for π-backdonation generally pose a
problem in all CO-based methodologies.
In order to exploit this improved accuracy of measurement
and in search for suitable ligand precursors for mesoionic pyra-
zolin-4-ylidenes,9 we decided to explore the donor strength of a
series of substituted pyrazoles bearing a variety of halogen,
alkyl, and aryl substituents as well as exocyclic O- and N-donor
atoms.
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, 117543 Singapore, Singapore. E-mail: chmhhv@nus.edu.sg;
Fax: +65 6779 1691; Tel: +65 6516 2670
†Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra, experimental procedures, CIF files for complexes 17, 20,
21 and 25. CCDC 870361–870364. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt30526g
8600 | Dalton Trans., 2012, 41, 8600–8608
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 1 Halogenation of the pyrazole scaffold
Entry
Substrate
R
X
Product
Yield
a
1
2
3
4
5
6
5
5
5
4
4
4
Me
Me
Me
Ph
Ph
Ph
Cl
Br
I
Cl
Br
I
7
8
9
10
11
12
—
96%
95%
>99%
>99%
92%
Scheme 1 Synthesis of pyrazoles.
a Only side-chain halogenation observed.
Scheme 2 Halogenation under ultrasound irradiation (X = Cl, Br, I).
In a further step, we assessed the nucleophilicity of the
N-donor by reacting the free ligand with various electrophiles.
Most nucleophilicity parameters rely either on DFT calculations
or more often on kinetic measurements.10 These scales allow for
a quantitative and precise comparison of a multitude of nucleo-
philes, but the determination of values calls for precise measure-
ments of kinetic data or extensive calculations. Our simple
experiment might close the gap in the methodology, if only a
quick and qualitative determination of nucleophilicity is needed.
Scheme 3 Synthesis of donor-functionalized pyrazoles.
In order to introduce greater diversity into the series of tested
pyrazoles, we included donor-substituted pyrazoles. Unfortu-
nately, the synthesis of this class of compounds is less straight-
forward than the procedure for their alkyl- or aryl-substituted
counterparts. While the reaction of 1,3-diketones with phenyl-
hydrazine hydrochloride readily yields the corresponding pyra-
zoles, a similar reaction with β-ketoesters or β-ketoamides leads
to the formation of pyrazolin-5-ones, along with a small amount
of the desired pyrazoles with donor-functionalities in the 5-pos-
ition.16 A synthetically simple method for the formation of
5-aminopyrazoles published by Dodd and Martinez, who per-
formed the cyclisation reaction in the presence of Lawesson’s
reagent, thus generating a β-ketothioamide in situ, failed to give
the desired products in our syntheses.17 An attempt to extend
their methodology to the conversion of β-ketoesters proved
equally futile. We therefore sought to use the commercially
available 1-phenyl-3-methyl-pyrazolin-5-one 13 as a starting
material.
Results and discussion
Pyrazoles
Since the discovery of the Knorr pyrazole synthesis, several
procedures for the synthesis of pyrazoles have emerged due to
the widespread application of substituted pyrazoles in medicinal
chemistry and as ligands in coordination chemistry.11,12 Drawing
from these procedures, the desired pyrazoles were made. Starting
from commercially available 1,3-diketones, a condensation reac-
tion with phenylhydrazine hydrochloride gave the desired alkyl
and aryl pyrazoles in good yields (Scheme 1). Pure products
were obtained either by recrystallization (4) or by distillation of
the crude mixture (5 and 6).
The introduction of a halogen substituent in the 4-position on
the pyrazole scaffold is possible by treating the compound with
bromine or iodine directly.13 However, prolonged reaction times
of several days and harsh reaction conditions were needed in
order to reach full conversion of the starting materials. We there-
fore turned to a mild and efficient procedure described by Pereira
et al. who described the halogenation of pyrazoles by N-halosuc-
cinimides under ultrasound irradiation.14 These conditions gener-
ally yielded the desired halogenated pyrazoles in excellent yields
(Scheme 2). In the case of 1-phenyl-3,5-dimethyl-1H-pyrazole
(5) however, the reaction with N-chlorosuccinimide gave a
complex mixture of side-chain halogenated compounds, which is
in contrast with the findings of Pereira and co-workers. On the
other hand, the bromination and iodination reaction of 5 pro-
ceeded without any such complication and gave the desired
products exclusively (Table 1).15
The direct conversion of 13 to 14 can be effected by heating it
in the presence of a dehydration agent and anilinium chloride.18
Making use of the known advantages of microwave accelerated
chemistry, we could further improve this procedure, and were
able to obtain 14 in satisfactory yield. Finally, the methoxy func-
tionalized pyrazole 15 was obtained from 13 by Mitsunobu con-
densation (Scheme 3).19,20
Palladium complexes and donor strength determination
The 13C chemical shift of the carbene carbon in complexes of
the type trans-dibromo(1,3-diisopropylbenzimidazolin-2-ylidene)-
(pyrazole)palladium(II) 17–26 was used as a measure of donor
strength. In these complexes, a higher donor ability of the
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8600–8608 | 8601

View Article Online
Scheme 4 Cleavage of the dimeric palladium precursor complex.
Table 2 Synthesis of pyrazole complexes and 13Ccarbene NMR shifts
transoid pyrazole ligand is bound to decrease the Lewis acidity
of the metal centre, which in turn leads to a downfield shift of
the NMR resonance for the carbene carbon.6,7 The desired com-
plexes can be prepared in analogy to similar complexes prepared
previously in our group by cleavage of the dimeric species 16
(Scheme 4).8,21
Complexes 17–26 could be obtained from 16 in high to
quantitative yields without the necessity of any purification step.
All compounds are light yellow solids that are readily soluble in
chlorinated organic solvents, but insoluble in diethyl ether and
alkanes.
Entry
R1
R2
X
Product
Yield
δ Ccarbene (ppm)
1
2
3
4
5
6
7
8
9
10
Me
Ph
Me
Ph
H
H
H
Br
I
Cl
Br
I
17
18
19
20
21
22
23
24
25
26
87%
95%
163.0
161.6
163.1
161.8
161.8
160.9
160.8
160.9
162.6
162.8
iPr
iPr
>99%
98%
>99%
>99%
>99%
98%
Me
Me
Ph
Ph
Ph
OMe
NHPh
Me
Me
Ph
Ph
Ph
Me
Me
H
H
>99%
>99%
Upon cleavage of 16, the isopropyl moieties in the NHC
ligand are no longer spectroscopically equal, and thus two sets
1
of isopropyl signals are present in the H NMR spectra. Com-
pared to the starting material, where the isopropyl methine
proton resonates at 6.54 ppm, a pronounced upfield shift of up to
1.4 ppm is observed. It is noteworthy that in complexes 17–26,
the methine proton resonance varies between 5.07 and
6.26 ppm, with one of the two resonances being pronouncedly
more upfield shifted than the other. The phenyl substituent on
the pyrazole ligand exerts a shielding influence on one methine
proton, leading to the observed change in chemical shift. A
similar aryl substituent-induced upfield shift in complexes with a
hindered rotation along the metal–ligand axes has been observed
by us previously.22 The resonance for the methyl groups of the
isopropyl moiety is less affected, with a comparatively small
upfield shift of ∼0.4 ppm and a smaller gap between the two
resonances. The ring protons in the pyrazole ring were shifted
downfield by ∼0.1 ppm in the case of the simple pyrazoles 4–6,
while the donor-substituted pyrazoles showed an upfield shift of
∼0.3 ppm. While the former can be attributed to the deshielding
effect of complexation, the latter could be caused by a stronger
mesomeric effect in the coordinated pyrazoles.
Fig. 1 13C NMR resonances of Ccarbene in 17–26.
good agreement with predictions based on the inductive effects
of the various substituents. In the case of pyrazoles without a
halogen substituent in the 4-position, the diisopropyl substituted
pyrazole shows the best donor abilities, followed by the dimethyl
substituted system, and the diphenyl substituted system exhibi-
ting the lowest donor strength (entries 1–3). This is in line with a
i
decreasing + I effect of the substituents in the order Pr > Me >
The observed 13C NMR chemical shifts of the carbene carbon
in these complexes differ in a narrow range of 2.3 ppm, with
complex 23 exhibiting the lowest value of δ = 160.8 ppm and
complex 19 the highest value of δ = 163.1 ppm (Table 2, entries
3 and 7). Nevertheless, all complexes can be distinguished
unambiguously by their characteristic NMR shifts (Fig. 1). The
obtained values are in good agreement with previously observed
chemical shifts for similar complexes. For example, an imidazole
analogue of complexes 17–26 exhibits a chemical shift of the
carbene carbon of δ = 161.4 ppm, while a complex bearing a
more weakly donating pyridine ligand was found to give rise to
a resonance signal at δ = 160.0 ppm.6
Ph. The N- and O-substituted pyrazoles have an intermediate
donor strength in this series, reflecting the competing influences
of −I and +M effect exerted by these substituents.
The introduction of halogen substituents generally leads to an
upfield shift of the carbene carbon resonance in the case of both
dimethyl and diphenyl substituted pyrazoles. The observed
upfield shift is greater for a more electronegative bromo- com-
pared to an iodo-substituent. A notable exception is entry 6. The
chloro-substituted pyrazole complex has a carbene carbon reson-
ance at δ = 160.9 ppm, while the bromo-derivative has the lower
value of δ = 160.8 ppm (entries 6 and 7) implying that the
chloropyrazole may be a slightly stronger donor. At first sight, it
seems that our system is not able to reflect the electronic proper-
ties of these two halopyrazoles with absolute certainty. However,
The observed chemical shifts, and thus the experimentally
determined donor strengths of the substituted pyrazoles are in
8602 | Dalton Trans., 2012, 41, 8600–8608
This journal is © The Royal Society of Chemistry 2012

View Article Online
Fig. 2 Molecular structures of 17 and 25 (hydrogen atoms have been
omitted for clarity, thermal ellipsoids at 50% probability).
Fig. 3 Molecular structures of 20 and 21 (hydrogen atoms and solvent
molecules have been omitted for clarity, thermal ellipsoids at 50%
probability).
the different contributions of −I versus +M effects in Br versus
Cl may offer an explanation for this observation. Although the
Cl atom has a greater −I effect compared to Br, its smaller
size may offer better orbital overlap with carbon enhancing its
+M effect, which in turn can compensate its −I effect. For the
bigger Br and I atoms, a smaller contribution of the +M effect is
expected. Taking this into consideration, there is good agreement
with the theoretical predictions of donor strength and the experi-
mentally determined values.
The Pd–Ccarbene bond distances fall into the narrow range of
1.943(5) Å to 1.953(6) Å, while Pd–N bonds show marginally
more variation and range from 2.090(4) Å to 2.108(5) Å. These
values are in good agreement with bond lengths in similar
systems. For instance, with a pyridine ligand in the trans-
position, the Pd–Ccarbene bond was found to be 1.953(4) Å long,
and the Pd–N bond 2.113(3) Å, while imidazole and methyl-
imidazole gave Pd–Ccarbene bond lengths of 1.943(2) Å and
1.952(3) Å, as well as 2.097(2) Å and 2.105(3) Å for the Pd–N
bonds.6,7c
Crystal structures
Alkylation experiments
Single crystals suitable for X-ray diffraction studies were
obtained for an exemplary selection of complexes by slow diffu-
sion of diethyl ether in a saturated chloroform solution. Fig. 2
and 3 depict the molecular structures of complexes 17 and 25,
and 20 and 21, respectively. Crystallographic data is given in
Table 3 and selected bond parameters are listed in table, and a
selection of crystallographic data is given in Table 4.
In each case, palladium has a slightly distorted square-planar
coordination geometry, in which the NHC and the pyrazole are
trans to each other. This is in line with the observations made on
previously synthesized systems, and reflects the tendency to
avoid a sterically more crowded cis-configuration. The planes of
the benzimidazolin-2-ylidene and the pyrazole ligand are almost
parallel, with small dihedral angles ranging from 5–14°. The
phenyl substituent on N(4) is twisted out of the pyrazole plane
by 50–64° in order to avoid unfavourable steric interaction with
the isopropyl groups on the NHC ligand.
To determine the approximate nucleophilicity of the substituted
pyrazoles, the reactivity with three electrophiles of increasing
strength was tested (Scheme 5). Trimethyloxonium tetrafluoro-
borate is an extremely strong alkylation agent, and it was
expected that it would alkylate all pyrazoles irrespective of the
electron density at N2. Ethyl iodide, while still being a strong
alkylation agent, is nevertheless somewhat less electrophilic than
Meerwein’s salt, and ethyl bromide is even less reactive. Based
on previously developed procedures, the pyrazoles were sub-
jected to test reactions by heating them in an excess of neat ethyl
halide, or by exposing them to trimethyloxonium tetrafluoro-
borate in anhydrous dichloromethane.
As anticipated, all pyrazoles under scrutiny with the exception
of 14 were smoothly alkylated by trimethyloxonium tetrafluoro-
borate, albeit no correlation between the yields of pyrazolium
salts and the determined donor strength of their parent pyrazoles
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8600–8608 | 8603

View Article Online
Table 3 Selected crystallographic data, data collection and refinement parameters
17
25
20·CHCl3
21·CHCl3
Formula
C24H30Br2N4Pd
640.76
Yellow plates
0.50 × 0.30 × 0.06
Orthorhombic
Pbca
9.0480(3)
16.4781(6)
35.4180(14)
90
C24H30Br2ON4Pd
656.75
Yellow needles
0.60 × 0.20 × 0.08
Orthorhombic
Pbca
9.0465(6)
16.5328(10)
35.937(2)
90
C25H30Br3Cl3N4Pd
839.01
Yellow needles
0.50 × 0.26 × 0.10
Orthorhombic
P212121
11.706(4)
15.947(5)
16.887(6)
90
C25H30Br2Cl3IN4Pd
886.00
Yellow needles
0.26 × 0.22 × 0.16
Orthorhombic
P212121
11.741(2)
15.964(3)
16.643(3)
90
Mw (g mol−1
Colour, habit
)
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
90
90
5280.6(3)
8
1.612
90
90
5374.9(6)
8
1.623
223(2)
0.71073
2.27 to 27.50
6171
90
90
90
90
γ (°)
V (Å3)
3152.3(18)
4
1.768
223(2)
0.71073
1.76 to 27.50
7213
3119.5(10)
4
1.886
100(2)
0.71073
1.77 to 27.50
7164
Z
ρcalculated (g cm−3
Temperature (K)
Wavelength (Å)
θ (°)
)
223(2)
0.71073
1.15 to 27.49
6056
No. of unique reflections
Max. and min. transmission 0.8064, 0.2559
0.7569, 0.2158
0.6526, 0.2037
0.5379, 0.3925
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0647, wR2 = 0.1242 R1 = 0.0697, wR2 = 0.1552 R1 = 0.0538, wR2 = 0.1173 R1 = 0.0360, wR2 = 0.0930
R1 = 0.0845, wR2 = 0.1314 R1 = 0.0987, wR2 = 0.1648 R1 = 0.0953, wR2 = 0.1337 R1 = 0.0392, wR2 = 0.0945
Goodness-of-fit on F2
1.199
1.141
2.679 and −1.165
0.943
1.544 and −0.686
1.068
1.342 and −1.000
Peak/hole (e Å−3
)
1.107 and −1.129
Table 4 Selected interatomic distances (Å) and angles (°)
17
25
20·CHCl3
21·CHCl3
Pd(1)–C(1)
Pd(1)–Br(1)
Pd(1)–Br(2)
Pd(1)–N(3)
C(1)–Pd(1)–Br(1)
C(1)–Pd(1)–Br(2)
Br(1)–Pd(1)–N(3)
Br(2)–Pd(1)–N(3)
C(1)–Pd(1)–N(3)
Br(1)–Pd(1)–Br(2)
N(2)–C(1)–N(3)–N(4)
N(3)–N(4)–C(19)–C(20)
1.943(5)
2.4277(7)
2.4266(7)
2.090(4)
87.97(14)
87.33(14)
92.95(13)
91.76(13)
178.93(19)
174.75(3)
13.89
1.953(6)
1.950(8)
2.4159(12)
2.4164(12)
2.107(6)
88.4(2)
1.947(5)
2.4066(8)
2.4063(8)
2.097(4)
88.21(15)
88.52(16)
91.35(12)
91.94(13)
179.5(2)
175.25(3)
5.33
2.4346(10)
2.4251(10)
2.108(5)
87.79(19)
87.76(19)
91.36(17)
93.13(17)
178.5(3)
174.94(4)
14.08
87.6(2)
90.97(17)
93.09(17)
179.3(3)
175.40(4)
10.09
64.42
49.89
58.55
58.05
could be observed. The unsystematic variation in yields can be
attributed to losses during the often difficult work-up of the
crude alkylation product, which was usually obtained along with
oily byproducts of unknown nature. Washing steps were required
to obtain analytically pure products, and slightly different
properties of the different pyrazolium salts can explain the
unpredictable losses in yield during this process. In the extreme
case of pyrazole 14 with an aminophenyl substituent in 5-pos-
ition, only a black oil could be obtained, from which no pyrazo-
lium salt could be isolated.
Scheme 5 Alkylation of pyrazoles.
No clear trend in yield was observed as well for the alkylation
with ethyl iodide, albeit more electron-rich pyrazoles gave some-
what higher yields than electron-poor systems. Notably, the four
most electron-deficient pyrazoles 4 and 10–12 were not alkylated
despite the harsh conditions, long reaction times, and large
excess of alkylating agent (Table 5, entries 1–4). Apparently,
these four pyrazoles can only be alkylated by the strongest elec-
trophile, which in turn underscores their low nucleophilicity.
In the case of the 5-methoxypyrazole 15 (entry 7), no alkyla-
transesterification product could be isolated, in which the
methoxy group had been replaced by an ethoxy group.
The weakest electrophile, ethyl bromide, showed no reaction
with most pyrazoles. Only the two most electron-rich pyrazoles
gave small to trace amounts of alkylation product, but even after
heating to reflux for 3 days, only partial conversion was
observed. In the case of 6, which gave a lower yield than 4, the
steric influence exerted by the bulky isopropyl substituent in the
α-position to the nucleophilic nitrogen certainly had an adverse
effect on the reactivity.
tion was observed. Instead, only small amounts of
a
8604 | Dalton Trans., 2012, 41, 8600–8608
This journal is © The Royal Society of Chemistry 2012

View Article Online
Based on these alkylation experiments, the pyrazoles can be
divided into three groups. Fig. 4 shows that there is a good corre-
lation between their nucleophilicity and their donor strength as
determined by 13C NMR spectroscopy. From this consistent
agreement, it is reasonable to assume that there exists an inter-
dependence between the donor strength – and thus, the electron
density at N2 – and the transition state energy for alkylation
reactions.
Pyrazoles that give rise to a carbene carbon resonance in
trans-[PdBr2(iPr2-bimy)(pyrazole)] upfield from 161.6 ppm are
the weakest donors in the series under investigation and can only
be alkylated using Meerwein’s salt. If the carbene carbon chemi-
cal shift falls in the range of 161.6–162.9 ppm, which corre-
sponds to an intermediate donor strength, both ethyl iodide and
trimethyloxonium tetrafluoroborate react readily with the pyra-
zole, and if the chemical shift is even higher, even ethyl bromide
can alkylate the pyrazoles.
Conclusion
In conclusion, we have been able to demonstrate that even subtle
variations of donor strength in pyrazole ligands can be detected
with great accuracy by our previously developed electronic para-
meter based on the 13C NMR chemical shift of the carbene
carbon in trans-[PdBr2(iPr2-bimy)L] complexes. A good corre-
lation between these experimentally determined donor strength
values and the nucleophilicity of the nitrogen donor atom in the
pyrazoles has been observed in alkylation experiments. This
indicates that our methodology is useful not only for donor
strength determinations, but also for semi-quantitative predic-
tions of the reactivity of nucleophiles. While kinetic measure-
ments give accurate information about the nucleophilicity of a
reactive centre, these measurements are not easily performed. In
contrast, our method is fast, safe and non-destructive, and the
obtained information is of sufficient accuracy in many cases.
Further investigations should focus on nucleophiles other than
pyrazoles, as well as on other electrophiles. Of special interest is
the extension of the reactivity scale towards more reactive
nucleophiles, and thus, less reactive alkylating agents.
Table 5 Alkylation experiments
Entry
Nucleophile
Electrophile
EtBr
EtI
Me3OBF4
Experimental section
1
2
3
4
5
6
7
8
9
11
10
12
5
8
9
15
14
4
0%
0%
0%
0%
0%
0%
0%
0%
0%
0%
70%
37%
67%
55%
51%
80%
General considerations
0%
All reactions were carried out without precautions to exclude air
and moisture, unless stated otherwise. Solvents were used as
24%b
50%a
received or dried using standard procedures. H and 13C NMR
1
Decompositionc
58%
62%
0%
Decompositiond
77%
spectra were recorded on a Bruker AV 300 spectrometer. The
chemical shifts are given relative to tetramethylsilane with the
solvent serving as internal standard for calibration (CHCl3, δ =
7.26 ppm [1H] and δ = 77.70 ppm [13C]). Mass spectra were
measured using a Finnigan LCQ spectrometer (ESI). Elemental
analyses were performed by the Chemical, Materials and Mo-
lecular Analysis Centre at the Department of Chemistry, National
7%b
Traceb
99%
75%
10
6
71%
a From ref. 28. b Obtained as a mixture of product and starting material;
yield determined by 1H NMR spectroscopy. c Methoxy group was
cleaved under reaction conditions, no alkylation product obtained.
d Black polymeric material was obtained.
Fig. 4 Correlation between the electronic parameter and the reactivity towards electrophiles.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8600–8608 | 8605

View Article Online
3
3
University of Singapore. Di-μ-bromo-bis(1,3-diisopropylbenz-
imidazolin-2-ylidene)-dibromo-dipalladium(II) (16) was prepared
according to a literature procedure.8
(d, JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.43 (d, JH–H = 7.0 Hz,
6 H, CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 163.0
(NCN), 150.1 (NvC), 143.3 (N–C), 138.8 (Ar–C), 134.0 (Ar–
C), 133.9 (Ar–C), 130.6 (2 × Ar–C), 129.7 (Ar–C), 129.4
(2 × Ar–C), 122.6 (2 × Ar–C), 113.1 (Ar–C), 113.0 (Ar–C),
108.0 (CH), 54.9 (C(CH3)2), 54.4 (C(CH3)2), 21.4 (C(CH3)2),
20.8 (C(CH3)2), 15.3 (CH3), 12.7 (CH3). Anal. Calcd for
C24H30Br2N4Pd: C, 44.99; H, 4.72; N, 8.74. Found: C, 44.77;
H, 4.58; N, 8.21. MS (ESI): m/z = 560 [M − Br]+, 663
[M + Na]+.
Syntheses
4-Chloro-1,3,5-triphenyl-1H-pyrazole (10). 1,3,5-Triphenyl-
1H-pyrazole (4) (500 mg, 1.68 mmol, 1.00 eq) and N-chlorosuc-
cinimide (561 mg, 4.20 mmol, 2.50 eq) were dissolved in ethyl
acetate (8 mL). The solution was sonicated at ambient tempera-
ture for 4 h. The reaction mixture was washed with saturated
sodium thiosulfate (2 × 20 mL) and water (20 mL), dried over
sodium sulfate and filtered. The solvent was removed in vacuo.
The product was obtained as an orange solid (577 mg, >99%
trans-Dibromo(1,3,5-triphenyl-1H-pyrazole)(1,3-diisopropyl-
benzimidazolin-2-ylidene)palladium(II) (18). Complex 18 was
prepared using 1,3,5-triphenyl-1H-pyrazole (4) (30 mg,
0.10 mmol, 2.0 eq). 73 mg of a pale yellow solid were obtained
(95% yield). 1H NMR (300 MHz, CDCl3): δ 8.41–8.48 (m, 2 H,
Ar–H), 7.86–7.94 (m, 2 H, Ar–H), 7.56–7.63 (m, 5 H, Ar–H),
7.36–7.47 (m, 2 H, Ar–H), 7.22–7.31 (m, 5 H, Ar–H),
7.07–7.14 (m, 2 H, Ar–H), 6.76 (s, 1 H, CH), 5.93 (sept,
1
yield). H NMR (300 MHz, CDCl3): δ 8.03 (m, 2 H, Ar–H),
7.27–7.52 (m, 13 H, Ar–H). 13C{1H} NMR (75 MHz, CDCl3):
δ 148.8 (CvN), 141.0 (C–N), 140.5 (Ar–C), 132.3 (Ar–C),
130.7 (2 × Ar–C), 129.6 (Ar–C), 129.6 (2 × Ar–C), 129.2
(2 × Ar–C), 129.2 (Ar–C), 129.1 (2 × Ar–C), 129.1 (Ar–C),
129.0 (Ar–C), 128.4 (2 × Ar–C), 128.3 (Ar–C), 125.5 (C–Cl,
Ar–C). Anal. Calcd for C21H15BrN2·HCl: C, 68.68; H, 4.39; N,
7.63. Found: C, 68.66; H, 3.93; N 7.68. MS (ESI): m/z = 331
[M + H]+.
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 5.07 (sept, JH–H = 7.0 Hz,
3
3
6 H, CH(CH3)2), 1.55 (d, JH–H = 7.0 Hz, 6 H, CH(CH3)2),
1.41 (d, JH–H = 7.0 Hz, 6 H, CH(CH3)2). 13C{1H} NMR
3
(75 MHz, CDCl3): δ 161.6 (NCN), 147.9 (N–C), 154.9 (NvC),
139.4 (Ar–C), 134.0 (Ar–C), 133.9 (Ar–C), 133.2 (Ar–C), 131.6
(2 × Ar–C), 130.4 (2 × Ar–C), 129.9 (Ar–C), 129.6 (Ar–C),
129.5 (Ar–C), 129.4 (2 × Ar–C), 129.3 (Ar–C), 129.3 (2 ×
Ar–C), 129.1 (2 × Ar–C), 128.8 (2 × Ar–C), 122.6 (Ar–C),
122.5 (Ar–C), 113.0 (Ar–C), 113.0 (Ar–C), 107.8 (CH), 54.7
(C(CH3)2), 54.3 (C(CH3)2), 20.9 (C(CH3)2), 20.9 (C(CH3)2).
Anal. Calcd for C34H34Br2N4Pd: C, 53.39; H, 4.48; N, 7.32.
Found: C, 53.15; H, 4.27; N, 7.31.
5-Aminophenyl-3-methyl-1-phenyl-1H-pyrazole (14). 3-Methyl-
1-phenyl-1H-pyrazol-5-one (13) (600 mg, 3.44 mmol, 1.00 eq),
phosphorous pentoxide (293 mg, 2.06 mmol, 0.60 eq) and ani-
linium hydrochloride (535 mg, 4.13 mmol, 1.10 eq) were
thoroughly mixed and irradiated in a microwave at 210 °C for
30 min. The black solid formed during the reaction was dis-
solved in water and ethanol (1 : 1, 10 mL). The aqueous phase
was extracted with ethyl acetate (2 × 50 mL). The combined
organic layers were dried over natrium sulfate, filtered and the
solvent was evaporated in vacuo. Recrystallization of the residue
from hexane–ethyl acetate yielded the product as a dark orange
trans-Dibromo(3,5-diisopropyl-1-phenyl-1H-pyrazole)(1,3-diiso-
propylbenzimidazolin-2-ylidene)palladium(II) (19). Complex 19
was prepared using 3,5-diisopropyl-1-phenyl-1H-pyrazole (6)
(23 mg, 0.10 mmol, 2.0 eq). 70 mg of a yellow solid were
obtained (quantitative yield). 1H NMR (300 MHz, CDCl3):
δ 7.75–7.83 (m, 2 H, Ar–H), 7.59–7.68 (m, 3 H, Ar–H),
7.46–7.53 (m, 1 H, Ar–H), 7.39–7.45 (m, 1 H, Ar–H),
7.08–7.16 (m, 2 H, Ar–H), 6.29 (sept, 3JH–H = 7.0 Hz, 6 H, CH-
(CH3)2), 6.09 (s, 1 H, CH), 5.10 (sept, 3JH–H = 7.0 Hz, 6 H, CH-
(CH3)2), 3.98 (sept, 3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 2.77 (sept,
1
solid (522 mg, 64%). H NMR (300 MHz, CDCl3): δ 7.53–7.58
(m, 2 H, Ar–H), 7.44–7.59 (m, 2 H, Ar–H), 7.27–7.39 (m, 4 H,
Ar–H), 6.59–7.00 (m, 2 H, Ar–H), 6.01 (s, 1 H), 5.63 (br, 1 H,
NH), 2.35 (s, 3 H, CH3). The analytical data was in accordance
with the reported values.23
General procedure for the preparation of trans-dibromo-(1,3-
diisopropylbenzimidazolin-2-ylidene)(pyrazole)palladium(II)
complexes. The respective pyrazole (0.10 mmol, 2.0 eq) was
added to a solution of di-μ-bromo-bis(1,3-diisopropylbenzimida-
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.76 (d, JH–H = 7.0 Hz, 6 H,
3
3
CH(CH3)2), 1.54 (d, JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.42 (d,
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.12 (d, JH–H = 7.0 Hz, 6 H,
3
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 163.1 (NCN),
163.3 (CvN), 154.1, (C–N), 139.0 (Ar–C), 134.0 (Ar–C), 133.9
(Ar–C), 131.4 (2 × Ar–C), 129.8 (Ar–C), 129.3 (2 × Ar–C),
122.6 (Ar–C), 122.5 (Ar–C), 113.0 (Ar–C), 113.0 (Ar–C), 100.5
(CH), 54.9 (C(CH3)2), 54.3 (C(CH3)2), 29.1 (C(CH3)2), 26.4
(C(CH3)2), 23.4 (C(CH3)2), 23.1 (C(CH3)2), 21.3 (C(CH3)2),
20.9 (C(CH3)2). Anal. Calcd for C28H39Br2N4Pd: C, 48.26; H,
5.50; N, 8.04. Found: C, 48.48; H, 4.99; N 8.06.
zolin-2-ylidene)-dibromo-dipalladium(II)
(16)
(47
mg,
0.05 mmol, 1.0 eq) in dichloromethane (4 mL) and the resulting
mixture was stirred at ambient temperature for 30 min. Then, the
solvent was removed under reduced pressure and the remaining
solid dried in vacuo.
trans-Dibromo(3,5-dimethyl-1-phenyl-1H-pyrazole)(1,3-diiso-
propylbenzimidazolin-2-ylidene)palladium(II) (17). Complex 17
was prepared using 3,5-dimethyl-1-phenyl-1H-pyrazole (5)
(17 mg, 0.10 mmol, 2.0 eq). 56 mg of a pale yellow solid were
obtained (87% yield). 1H NMR (300 MHz, CDCl3): δ 7.73–7.83
(m, 2 H, Ar–H), 7.56–7.71 (m, 3 H, Ar–H), 7.39–7.55 (m, 2 H,
trans-Dibromo(4-bromo-3,5-dimethyl-1-phenyl-1H-pyrazole)(1,3-
diisopropylbenzimidazolin-2-ylidene)palladium(II) (20). Complex
20 was prepared using 4-bromo-3,5-dimethyl-1-phenyl-1H-pyra-
zole (8) (25 mg, 0.10 mmol, 2.0 eq). 70 mg of a pale yellow
3
Ar–H), 7.08–7.18 (m, 2 H, Ar–H), 6.25 (sept, JH–H = 7.0 Hz,
1 H, CH(CH3)2), 6.08 (s, 1 H, CH), 5.27 (sept, JH–H = 7.0 Hz,
3
1
solid were obtained (98% yield). H NMR (300 MHz, CDCl3):
1 H, CH(CH3)2), 2.75 (s, 3 H, CH3), 2.15 (s, 3 H, CH3), 1.77
8606 | Dalton Trans., 2012, 41, 8600–8608
δ 7.61–7.82 (m, 5 H, Ar–H), 7.39–7.54 (m, 2 H, Ar–H),
This journal is © The Royal Society of Chemistry 2012

View Article Online
7.08–7.18 (m, 2 H, Ar–H), 6.22 (sept, 3JH–H = 7.0 Hz, 1 H, CH-
7.05–7.15 (m, 2 H, Ar–H), 5.61 (sept, 3JH–H = 7.0 Hz, 6 H, CH-
3
3
(CH3)2), 5.22 (sept, JH–H = 7.0 Hz, 1 H, CH(CH3)2), 2.76 (s,
(CH3)2), 5.10 (sept, JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.48 (d,
3 H, CH3), 2.17 (s, 3 H, CH3), 1.77 (d, 3JH–H = 7.0 Hz, 6 H, CH
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.40 (d, JH–H = 7.0 Hz, 6 H,
3
3
(CH3)2), 1.42 (d, JH–H = 7.0 Hz, 6 H, CH(CH3)2). 13C{1H}
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 160.8 (NCN),
152.9 (CvN), 145.7 (N–C), 139.1 (Ar–C), 133.9 (Ar–C), 133.8
(Ar–C), 131.9 (Ar–C), 131.4 (Ar–C), 131.2 (2 × Ar–C), 130.7
(2 × Ar–C), 130.3 (Ar–C), 130.1 (2 × Ar–C), 129.7 (Ar–C),
129.3 (2 × Ar–C), 129.0 (2 × Ar–C), 128.6 (2 × Ar–C), 128.0
(Ar–C), 122.6 (Ar–C), 122.6 (Ar–C), 113.0 (Ar–C), 113.0
(Ar–C), 96.8 (C–Br), 54.6 (C(CH3)2), 54.4 (C(CH3)2), 20.8
(C(CH3)2), 20.8 (C(CH3)2). Anal. Calcd for C34H33Br3N4Pd: C,
48.40; H, 3.94; N, 6.64. Found: C, 48.74; H, 4.16; N, 7.32.
NMR (75 MHz, CDCl3): δ 161.8 (NCN), 148.9 (NvC), 142.0
(N–C), 138.7 (Ar–C), 134.0 (Ar–C), 133.9 (Ar–C), 130.5 (2 ×
Ar–C), 130.2 (Ar–C), 129.6 (2 × Ar–C), 122.7 (2 × Ar–C),
113.1 (Ar–C), 113.0 (Ar–C), 97.2 (C–Br), 55.1 (C(CH3)2),
54.4 (C(CH3)2), 21.3 (C(CH3)2), 20.8 (C(CH3)2), 14.3 (CH3),
12.1 (CH3). Anal. Calcd for C24H29Br3N4Pd·CHCl3: C, 35.79;
H, 3.60; N, 6.68. Found: C, 36.05; H, 3.18; N, 6.68. MS (ESI):
m/z = 637 [M − Br]+.
trans-Dibromo(4-iodo-3,5-dimethyl-1-phenyl-1H-pyrazole)(1,3-
diisopropylbenzimidazolin-2-ylidene)palladium(II) (21). Complex
21 was prepared using 4-iodo-3,5-dimethyl-1-phenyl-1H-pyra-
zole (9) (30 mg, 0.10 mmol, 2.0 eq). 79 mg of a pale yellow
solid were obtained (quantitative yield). 1H NMR (300 MHz,
CDCl3): δ 7.75–7.81 (m, 2 H, Ar–H), 7.62–7.71 (m, 3 H,
Ar–H), 7.48–7.53 (m, 1 H, Ar–H), 7.41–7.46 (m, 1 H, Ar–H),
trans-Dibromo(4-iodo-1,3,5-triphenyl-1H-pyrazole)(1,3-diiso-
propylbenzimidazolin-2-ylidene)palladium(II) (24). Complex 24
was prepared using 4-iodo-1,3,5-triphenyl-1H-pyrazole (12)
(42 mg, 0.10 mmol, 2.0 eq). 87 mg of an off-white solid were
obtained (98% yield). 1H NMR (300 MHz, CDCl3): δ 8.15–8.23
(m, 2 H, Ar–H), 7.84–7.86 (m, 2 H, Ar–H), 7.49–7.69 (m, 6 H,
Ar–H), 7.31–7.45 (m, 7 H, Ar–H), 7.06–7.13 (m, 2 H, Ar–H),
3
3
7.11–7.17 (m, 2 H, Ar–H), 6.22 (sept, JH–H = 7.0 Hz, 1 H,
5.55 (sept, JH–H = 7.0 Hz, 6 H, CH(CH3)2), 5.11 (sept,
3
3
CH(CH3)2), 5.21 (sept, JH–H = 7.0 Hz, 1 H, CH(CH3)2), 2.78
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.47 (d, JH–H = 7.0 Hz, 6 H,
3
3
(s, 3 H, CH3), 2.20 (s, 3 H, CH3), 1.77 (d, JH–H = 7.0 Hz, 6 H,
CH(CH3)2), 1.40 (d, JH–H = 7.0 Hz, 6 H, CH(CH3)2). 13C{1H}
3
CH(CH3)2), 1.43 (d, JH–H = 7.0 Hz, 6 H, CH(CH3)2). 13C{1H}
NMR (75 MHz, CDCl3): δ 160.9 (NCN), 156.0 (NvC), 149.0
(N–C), 139.2 (Ar–C), 133.9 (Ar–C), 133.8 (Ar–C), 132.6
(Ar–C), 132.2 (2 × Ar–C), 131.1 (2 × Ar–C), 130.9 (2 × Ar–C),
130.1 (Ar–C), 130.0 (Ar–C), 129.7 (Ar–C), 129.3 (3 × Ar–C),
129.0 (2 × Ar–C), 128.6 (2 × Ar–C), 122.6 (Ar–C), 122.6 (Ar–
C), 113.0 (Ar–C), 113.0 (Ar–C), 65.8 (C–I), 54.6 (C(CH3)2),
54.4 (C(CH3)2), 20.8 (C(CH3)2), 20.8 (C(CH3)2). Anal. Calcd
for C34H33Br2IN4Pd: C, 45.84; H, 3.73; N, 6.29. Found: C,
45.93; H, 3.49; N, 6.57.
NMR (75 MHz, CDCl3): δ 161.8 (NCN), 152.0 (NvC),
145.4 (N–C), 139.0 (Ar–C), 134.1 (Ar–C), 133.9 (Ar–C), 130.6
(2 × Ar–C), 130.2 (Ar–C), 129.6 (2 × Ar–C), 122.7 (2 × Ar–C),
113.2 (Ar–C), 113.0 (Ar–C), 65.9 (C–I), 55.1 (C(CH3)2),
54.5 (C(CH3)2), 21.4 (C(CH3)2), 20.9 (C(CH3)2), 16.2 (CH3),
14.0 (CH3). Anal. Calcd for C24H29Br2IN4Pd·CHCl3: C, 33.89;
H, 3.41; N, 6.32. Found: C, 34.81; H, 3.19; N, 6.65. MS (ESI):
m/z = 685 [M − Br]+.
trans-Dibromo(4-chloro-1,3,5-triphenyl-1H-pyrazole)(1,3-diiso-
propylbenzimidazolin-2-ylidene)palladium(II) (22). Complex 22
was prepared using 4-chloro-1,3,5-triphenyl-1H-pyrazole (10)
(33 mg, 0.10 mmol, 2.0 eq). 80 mg of an orange solid were
obtained (quantitative yield). 1H NMR (300 MHz, CDCl3): δ
8.26–8.35 (m, 2 H, Ar–H), 7.82–7.89 (m, 2 H, Ar–H),
7.53–7.70 (m, 6 H, Ar–H), 7.31–7.45 (m, 7 H, Ar–H),
7.06–7.14 (m, 2 H, Ar–H), 5.67 (sept, 3JH–H = 7.0 Hz, 6 H, CH-
trans-Dibromo(5-methoxy-3-methyl-1-phenyl-1H-pyrazole)(1,3-
diisopropylbenzimidazolin-2-ylidene)palladium(II) (25). Complex
25 was prepared using 5-methoxy-3-methyl-1-phenyl-1H-pyrazole
(15) (19 mg, 0.10 mmol, 2.0 eq). 66 mg of a yellow solid were
obtained (quantitative yield). 1H NMR (300 MHz, CDCl3): δ
7.87–7.93 (m, 2 H, Ar–H), 7.59–7.66 (m, 2 H, Ar–H), 7.53–7.59
(m, 1 H, Ar–H), 7.48–7.53 (m, 1 H, Ar–H), 7.42–7.47 (m, 1 H,
3
Ar–H), 7.11–7.16 (m, 2 H, Ar–H), 6.26 (sept, JH–H = 7.0 Hz, 6
3
3
(CH3)2), 5.11 (sept, JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.50 (d,
H, CH(CH3)2), 5.60 (s, 1 H, CH), 5.43 (sept, JH–H = 7.0 Hz, 6
3
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.41 (d, JH–H = 7.0 Hz, 6 H,
H, CH(CH3)2), 3.84 (s, 1 H, OCH3), 2.76 (s, 1 H, CH3), 1.77 (d,
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 160.8 (NCN),
144.0 (N–C), 151.5 (CvN), 139.1 (Ar–C), 133.9 (Ar–C), 133.8
(Ar–C), 131.7 (2 × Ar–C), 131.3 (2 × Ar–C), 130.7 (Ar–C),
130.5 (2 × Ar–C), 130.1 (Ar–C), 130.1 (Ar–C), 129.8 (Ar–C),
129.4 (2 × Ar–C), 129.1 (2 × Ar–C), 128.7 (2 × Ar–C), 127.4
(Ar–C), 122.6 (Ar–C), 122.6 (Ar–C), 113.0 (Ar–C), 113.0
(Ar–C), 110.6 (C–Cl), 54.6 (C(CH3)2), 54.4 (C(CH3)2), 20.8
(C(CH3)2), 20.8 (C(CH3)2). Anal. Calcd for C34H33Br2ClN4Pd:
C, 51.09; H, 4.16; N, 7.01. Found: C, 51.29; H, 4.45; N, 7.09.
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.45 (d, JH–H = 7.0 Hz, 6 H,
3
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 162.3 (NCN),
157.5 (NvC), 151.0 (N–C), 137.2 (Ar–C), 134.1 (Ar–C), 133.9
(Ar–C), 129.7 (2 × Ar–C), 129.4 (Ar–C), 129.2 (2 × Ar–C),
122.6 (2 × Ar–C), 113.0 (Ar–C), 113.0 (Ar–C), 87.8 (CH), 59.4
(OCH3), 54.9 (C(CH3)2), 54.4 (C(CH3)2), 21.4 (C(CH3)2), 20.8
(C(CH3)2), 16.3 (CH3). Anal. Calcd for C24H30Br2N4OPd: C,
43.89; H, 4.60; N, 8.53. Found: C, 43.61; H, 4.15; N, 8.85. MS
(ESI): m/z = 616 [M − Br]+.
trans-Dibromo(4-bromo-1,3,5-triphenyl-1H-pyrazole)(1,3-diiso-
propylbenzimidazolin-2-ylidene)palladium(II) (23). Complex 23
was prepared using 4-bromo-1,3,5-triphenyl-1H-pyrazole (11)
(38 mg, 0.10 mmol, 2.0 eq). 84 mg of an orange solid were
obtained (quantitative yield). 1H NMR (300 MHz, CDCl3): δ
8.20–8.30 (m, 2 H, Ar–H), 7.81–7.88 (m, 2 H, Ar–H),
7.52–7.68 (m, 6 H, Ar–H), 7.32–7.44 (m, 7 H, Ar–H),
trans-Dibromo(5-aminophenyl-3-methyl-1-phenyl-1H-pyrazole)-
(1,3-diisopropyl-benzimidazolin-2-ylidene)palladium(II) (26).
Complex 26 was prepared using 5-aminophenyl-3-methyl-1-
phenyl-1H-pyrazole (14) (25 mg, 0.10 mmol, 2.0 eq). 75 mg of
a yellow solid were obtained (quantitative yield). 1H NMR
(300 MHz, CDCl3): δ 7.88–7.93 (m, 2 H, Ar–H), 7.60–7.69 (m,
3 H, Ar–H), 7.47–7.52 (m, 1 H, Ar–H), 7.41–7.46 (m, 1 H, Ar–H),
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8600–8608 | 8607

View Article Online
7.20–7.29 (m, 2 H, Ar–H), 7.10–7.15 (m, 2 H, Ar–H),
6.94–7.00 (m, 3 H, Ar–H), 6.26 (sept, 3JH–H = 7.0 Hz, 1 H, CH-
(CH3)2), 6.00 (s, 1 H, CH), 5.55 (sbr, 1 H, NH), 5.32 (sept,
3JH–H = 7.0 Hz, 1 H, CH(CH3)2), 2.75 (s, 3 H, CH3), 1.78 (d,
Notes and references
1 (a) O. Kühl, K. Lifson and W. Langel, Eur. J. Org. Chem., 2006, 2336;
(b) C. Crotti, E. Farnetti, S. Filipuzzi, M. Stener, E. Zangrando and
P. Moras, Dalton Trans., 2007, 133; (c) M. Mitoraj and A. Michalak,
Organometallics, 2007, 26, 6576; (d) J. Mathew, T. Thomas and C.
H. Suresh, Inorg. Chem., 2007, 46, 10800; (e) J. Mathew and C.
H. Suresh, Organometallics, 2011, 30, 1438.
2 (a) C. A. Tolman, J. Am. Chem. Soc., 1970, 92, 2953; (b) C. A. Tolman,
J. Am. Chem. Soc., 1970, 92, 2956; (c) C. A. Tolman, Chem. Rev., 1977,
77, 313.
3 (a) K. Denk, P. Sirsch and W. A. Herrmann, J. Organomet. Chem., 2002,
649, 219; (b) A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller and R.
H. Crabtree, Organometallics, 2003, 22, 1663; (c) R. A. Kelly III,
H. Clavier, S. Giudice, N. M. Scott, E. D. Stevens, J. Bordner,
I. Samardjiev, C. D. Hoff, L. Cavallo and S. P. Nolan, Organometallics,
2008, 27, 202.
3
3JH–H = 7.0 Hz, 6 H, CH(CH3)2), 1.44 (d, JH–H = 7.0 Hz, 6 H,
CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3) δ = 162.8 (NCN),
151.0 (CvN), 145.2 (C–N), 141.8 (C–N), 137.1 (Ar–C), 134.1
(Ar–C), 133.9 (Ar–C), 130.7 (Ar–C), 130.1 (Ar–C), 130.0 (2 × Ar–
C), 129.9 (2 × Ar–C), 122.7 (Ar–C), 122.6 (2 × Ar–C), 117.9 (2 ×
Ar–C), 113.0 (Ar–C), 113.0 (Ar–C), 94.5 (CH), 54.9 (C(CH3)2),
54.4 (C(CH3)2), 21.4 (C(CH3)2), 20.8 (C(CH3)2), 15.9 (CH3). Anal.
Calcd for C29H33Br2N5Pd: C, 48.52; H, 4.63; N, 9.76. Found: C,
48.15; H, 3.93; N, 9.40. MS (ESI): m/z = 636 [M − Br]+.
4 (a) A. B. P. Lever, Inorg. Chem., 1990, 29, 1271; (b) A. B. P. Lever,
Inorg. Chem., 1991, 30, 1980.
5 L. Perrin, E. Clot, O. Eisenstein, J. Loch and R. H. Crabtree, Inorg.
Chem., 2001, 40, 5806.
6 H. V. Huynh, Y. Han, R. Jothibasu and J. A. Yang, Organometallics,
2009, 28, 5395.
7 (a) M. V. Baker, P. J. Barnard, S. K. Brayshaw, J. L. Hickey, B.
W. Skelton and A. H. White, Dalton Trans., 2005, 37; (b) W.
A. Herrmann, O. Runte and G. J. Artus, J. Organomet. Chem., 1995,
501, C1; (c) Y. Han, H. V. Huynh and G. K. Tan, Organometallics, 2007,
26, 6447; (d) E. S. Chernyshova, R. Goddard and K.-R. Pörschke,
Organometallics, 2007, 26, 3236.
General procedure for the alkylation with bromoethane
The pyrazole was dissolved in bromoethane (2 mL) and the
resulting mixture was heated to 45 °C for 3 d. Then, the solvent
was removed under reduced pressure. The crude product was
washed with diethyl ether and dried in vacuo.
General procedure for the alkylation with iodoethane
The pyrazole was dissolved in iodoethane (2 mL) and the result-
ing mixture was heated to 90 °C for 3 d. Then, the solvent was
removed under reduced pressure. The crude product was washed
with diethyl ether and dried in vacuo.
8 H. V. Huynh, Y. Han, J. H. H. Ho and G. K. Tan, Organometallics, 2006,
25, 3267.
9 (a) Y. Han and H. V. Huynh, Dalton Trans., 2011, 40, 2141; (b) Y. Han,
L. J. Lee and H. V. Huynh, Organometallics, 2009, 28, 2778; (c) Y. Han,
L. J. Lee and H. V. Huynh, Chem.–Eur. J., 2010, 16, 771; (d) Y. Han and
H. V. Huynh, Chem. Commun., 2007, 1089; (e) Y. Han, H. V. Huynh and
G. K. Tan, Organometallics, 2007, 26, 6581.
10 (a) H. Mayr and M. Patz, Angew. Chem., 1994, 106, 990, (Angew. Chem.,
Int. Ed. Engl., 1994, 33, 938); (b) H. Mayr and A. R. Ofial,
Pure Appl. Chem., 2005, 77, 1807; (c) H. Mayr and A. R. Ofial,
J. Phys. Org. Chem., 2008, 21, 584; (d) T. W. Bentley, J. Phys. Org.
Chem., 2011, 24, 282; (e) S. Pratihart and S. Roy, Organometallics,
2011, 30, 3257.
11 (a) R. A. Singer, S. Caron, R. E. McDermott, P. Arpin and N. M. Do,
Synthesis, 2003, 1727; (b) J.-Y. Yoon, S.-G. Lee and H. Shin, Curr. Org.
Chem., 2011, 15, 657; (c) A. Schmidt and A. Dreger, Curr. Org. Chem.,
2011, 15, 1423.
12 N. Jonbert, E. Basle, M. Vaultier and M. Rucheault, Tetrahedron Lett.,
2010, 51, 2994.
13 (a) R. Hüttel, O. Schäfer and P. Jochum, Justus Liebigs Ann. Chem.,
1955, 593, 200; (b) M. I. Rodríguez-Franco, I. Dorrosonro, A.
I. Hernández-Higueras and G. Antequera, Tetrahedron Lett., 2001, 42,
863; (c) S. R. Stauffer, Y. Huang, C. J. Coletta, R. Tedesco and J.
A. Katzenellenbogen, Bioorg. Med. Chem., 2001, 9, 141.
14 H. A. Stefani, C. M. P. Pereira, R. B. Almeida, R. C. Braga, K. P. Guzen
and R. Cellac, Tetrahedron Lett., 2005, 46, 6833.
15 T. Sakamoto, Y. Kond, T. Suginome, S. Ohba and H. Yamanaka, Syn-
thesis, 1992, 552.
16 S. Guillou, F. J. Bonhomme and Y. L. Janin, Synthesis, 2008, 3504.
17 D. S. Dodd and R. L. Martinez, Tetrahedron Lett., 2004, 45, 4265.
18 A. N. Borisevich and P. S. Pel’kis, Chem. Heterocycl. Compd., 1969,
5(2), 312.
General procedure for the alkylation with trimethyloxonium
tetrafluoroborate
Trimethyloxonium tetrafluoroborate (1.2 eq) was suspended in
anhydrous dichloromethane (6 mL) under nitrogen atmosphere.
The pyrazole was added, and the mixture was heated to reflux for
15 h. The solvent was removed in vacuo, the residue taken up in
THF (0.5 mL), and the product was precipitated by addition of
diethyl ether (∼3 mL). It was isolated and dried in vacuo.
X-ray diffraction studies
X-ray data were collected with a Bruker AXS SMART APEX
diffractometer, using Mo Kα radiation at 100(2) K (21·CHCl3)
or 223(2) K (17, 20·CHCl3 and 25), with the SMART suite of
programs.24 Data were processed and corrected for Lorentz and
polarisation effects with SAINT,25 and for absorption effect with
SADABS.26 Structural solution and refinement were carried out
with the SHELXTL suite of programs.27 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. A summary of the most important crystallographic
data is given in Tables 3 and 4.
19 W. Holzer, B. Plagens and K. Lorenz, Heterocycles, 1997, 45(2), 309.
20 F. Palacios, A. M. Ochoa de Retana and J. Pagaldy, Tetrahedron, 1999,
55, 14451.
21 Y. Han and H. V. Huynh, Dalton Trans., 2009, 2201.
22 H. V. Huynh and J. Wu, J. Organomet. Chem., 2009, 694, 323.
23 Z. Kaleta, G. Tárkányi, Á. Gömöry, F. Káláman, T. Nagy and T. Sóos,
Org. Lett., 2006, 8, 1093.
24 SMART version 5.628, Bruker AXS Inc., Madison, WI, 2001.
25 SAINT+ version 6.22a, Bruker AXS Inc., Madison, WI, 2001.
26 G. W. Sheldrick, SADABS version 2.10, University of Göttingen, 2001.
27 SHELXTL version 6.14, Bruker AXS Inc., Madison, WI, 2000.
28 Y. Han, Group 10 transition metal chemistry of benzannulated/remote
N-heterocyclic carbene ligands, PhD thesis, National University of
Singapore, 2008.
Acknowledgements
We thank the National University of Singapore for financial
support (Grant No. R143-000-410-112 and SINGA scholarship)
and the CMMAC staff of the department of chemistry for techni-
cal assistance.
8608 | Dalton Trans., 2012, 41, 8600–8608
This journal is © The Royal Society of Chemistry 2012