1,2,3-triazolin-5-ylidenes: Synthesis of hetero-bis(carbene) Pd(II) complexes, determination of donor strengths, and catalysis

Article abstract of DOI:10.1021/om2010029

A series of hetero-bis(carbene) complexes trans-[PdBr₂( ⁱPr₂-bimy)(trz)] 1-4 (ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; trz = 1,2,3-triazolin-5-ylidene) bearing the constant ⁱPr₂-bimy and varying mesoionic 1,2,3-triazolin-5-ylidenes with different N-substituents has been synthesized as complex probes. Their ¹³C NMR spectroscopic evaluation shows that mesoionic 1,2,3-triazolin-5-ylidenes are, in general, stronger donors than classical NHCs, while weaker than some nonclassical NHCs, such as pyrazolin-3-ylidenes and mesoionic imidazolin-4-ylidenes. More importantly and for the first time, this methodology proves useful in establishing substituent effects in the donating abilities of 1,2,3-triazolin-5-ylidenes on a finer level. In addition, the trifluoroacetato analogues [Pd(O₂CCF ₃)₂(ⁱPr₂-bimy)(trz)] 5-7 have been synthesized through salt metathesis of 1, 2, and 4 with AgO₂CCF ₃. The catalytic activities of complexes 1, 2, and 4-7 were examined in the direct arylation of pentafluorobenzene. Complexes bearing less donating trz ligands perform better in this catalysis, and trifluoroacetato complexes outperformed their bromido analogues.

Full text of DOI:10.1021/om2010029

Article
pubs.acs.org/Organometallics
1,2,3-Triazolin-5-ylidenes: Synthesis of Hetero-bis(carbene) Pd(II)
Complexes, Determination of Donor Strengths, and Catalysis
Dan Yuan and Han Vinh Huynh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
S
* Supporting Information
ABSTRACT: A series of hetero-bis(carbene) complexes trans-[PdBr2(iPr2-
bimy)(trz)] 1−4 (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; trz =
i
1,2,3-triazolin-5-ylidene) bearing the constant Pr2-bimy and varying meso-
ionic 1,2,3-triazolin-5-ylidenes with different N-substituents has been synthe-
sized as complex probes. Their 13C NMR spectroscopic evaluation shows that
mesoionic 1,2,3-triazolin-5-ylidenes are, in general, stronger donors than classi-
cal NHCs, while weaker than some nonclassical NHCs, such as pyrazolin-3-
ylidenes and mesoionic imidazolin-4-ylidenes. More importantly and for the
first time, this methodology proves useful in establishing substituent effects in
the donating abilities of 1,2,3-triazolin-5-ylidenes on a finer level. In addition,
the trifluoroacetato analogues [Pd(O2CCF3)2(iPr2-bimy)(trz)] 5−7 have been
synthesized through salt metathesis of 1, 2, and 4 with AgO2CCF3. The
catalytic activities of complexes 1, 2, and 4−7 were examined in the direct
arylation of pentafluorobenzene. Complexes bearing less donating trz ligands perform better in this catalysis, and trifluoroacetato
complexes outperformed their bromido analogues.
redox behavior of many complexes due to either decomposition
or noninnocent ligand properties.10
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) have become standard ligands
in organometallic chemistry1 and have found wide application
in catalysis2 and many other fields.3 Recent NHC research has
gone beyond classical Arduengo-type carbenes,4 and more and
more emphasis has been put on the development of new
carbene types, which possess different topologies and electronic
properties.5 Numerous studies have been carried out to
examine their donating abilities, which influence the proper-
ties of pertinent transition-metal complexes to a great extent.
Thus, the knowledge of donating capacities may pave the way
for catalyst design. Although Tolman’s electronic parameter
(TEP)6 and related carbonyl-based methods are widely used to
determine the donor strengths of certain ligands, they suffer
from some limitations. For instance, the preparation of the
respective complexes {[Ni(CO)3L] or [MX(CO)2L] (L =
ligand in study, M = Ir(I) or Rh(I), X = halide)} requires the
handling of highly toxic starting materials, such as [Ni(CO)4]
or CO gases, or expensive Ir or Rh precursors. Furthermore,
their application for the study of NHCs gave inconsistent
results in certain cases,7 mainly due to the wrong assumption
that the π back-donation from metal centers to NHCs or other
ligands competing with that to the CO ligands is negligible.8
Indeed, caution must be taken when donating abilities of
ligands, and, in particular, those of similar ligands, are compared
based on wavenumbers.9 Another major approach known as
Lever’s electronic parameter (LEP) is based on electrochemical
E0 values of a redox couple of complexes bearing the ligands of
interest. Limitations to that method lie in the requirement for
less-common electrochemical apparatus and the nonreversible
To overcome these problems, we have recently introduced a
new, safe, and nondestructive method that employs a 13C NMR
spectroscopic evaluation of complexes of the type trans-
[PdBr2(iPr2-bimy)L] (iPr2-bimy = 1,3-diisopropylbenzimidazo-
lin-2-ylidene), in which L represents the ligand of interest.11 It
was empirically found that the 13C carbenoid signal of the
constant [PdBr2(iPr2-bimy)] probe was sensitive to the transoid
ligand L, whereby stronger donating ligands would lead to a
more downfield shift.12 The donor strengths of 25 Werner-type
and organometallic ligands have been compared on a unified
scale in the previous study.11
An extension of this methodology to other ligand systems,
especially to newly emerging and nonclassical carbenes, such as
1,2,3-triazolin-5-ylidenes (trz), is desirable. Following the first
report on complexes of this new type of mesoionic carbenes by
Albrecht et al,13 a number of studies have appeared on the
syntheses of related transition-metal complexes and of their
catalytic activities.14 Furthermore, the isolation and structural
characterization of the free trz ligand by Bertrand et al.15 pro-
vided more insights into the understanding of its mesoionic
nature. IR spectroscopic evaluation of their Ir(I) and Rh(I)
carbonyl complexes revealed that trz ligands are stronger donors
than classical NHCs, but weaker than mesoionic imidazolin-
4-ylidenes or pyrazolin-4-ylidenes. However, none of these
systems were capable in discerning any substituent effects, since
Received: October 18, 2011
Published: December 23, 2011
© 2011 American Chemical Society
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complexes of different carbenes with either alkyl or phenyl
substituents gave essentially the same CO stretching frequen-
cies, highlighting the above-mentioned limitations of carbonyl-
based methodologies.14b,15b
As our contribution to a more detailed understanding of
mesoionic trz ligands and in extension of our 13C NMR-based
electronic parameter, a series of Pd(II) hetero-bis(carbene)
complexes of the type trans-[PdBr2(iPr2-bimy)(trz)] have been
synthesized as spectroscopic probes for the donor strength
evaluation and comparison of 1,2,3-triazolin-5-ylidenes. The
preparation of trifluoroacetato analogues [Pd(O2CCF3)2(iPr2-
bimy)(trz)] and their catalytic activities in the direct arylation
of pentafluorobenzene are reported as well.
step depend largely on the N1 substituent. The reaction with
iPr-triazole gave a good yield of 88% after heating in neat benzyl
bromide overnight, whereas the yield dropped to 46% when the
same reaction condition was applied to Bn-triazole. Alkylation
of Ph-triazole gave only 32% yield even though the reaction
time was prolonged to 3 days. Apparently, the yields dropped
as the inductive effect of the N1 substituent decreases. In the
1H NMR spectra of salts A·HBr−D·HBr, downfield signals
ranging from 9.75 to 10.18 ppm were assigned to the protons
attached to the procarbene carbon atoms, supporting the
successful alkylations. Moreover, a base peak assignable to the
triazolium cation in each ESI mass spectrum also corroborates
the formation of the expected salts.
Synthesis of Pd(II) Complexes. To determine and com-
pare the donor strengths of 1,2,3-triazolin-5-ylidenes, hetero-
bis(carbene) complexes trans-[PdBr2(iPr2-bimy)(trz)] bearing
RESULTS AND DISCUSSION
■
Synthesis of the Ligand Precursors. Triazolium salts
with phenyl (A·HBr), benzyl (B·HBr, C·HBF4), or isopropyl
(D·HBr) N1 substituents were synthesized via two-step
reactions (Scheme 1). 1-Substituted 1,2,3-triazoles were first
i
the constant Pr2-bimy probe and the trz ligands in question
were synthesized. One-pot bridge-cleavage reactions of dimeric
[PdBr2(iPr2-bimy)]2 with 2 equiv of ligand precursors A·HBr,
B·HBr, or D·HBr and 1.2 equiv of Ag2O in CH2Cl2 yielded the
desired hetero-bis(carbene) complexes trans-[PdBr2(iPr2-
bimy)(A)] (1), trans-[PdBr2(iPr2-bimy)(B)] (2) and trans-
[PdBr2(iPr2-bimy)(D)] (4) suitable for measuring the donor
strengths of mesoionic 1,2,3-triazole-derived carbenes (Scheme 2).
Scheme 1. Preparation of Ligand Precursors A·HBr−D·HBr
−
Because ligand precursor C·HBF4 bears BF4 as the counter-
anion, complex trans-[PdBr2(iPr2-bimy)(C)] (3) was prepared
by reacting it with a tribromido complex, generated in situ by
addition of [N(n-Bu)4]Br to [PdBr2(iPr2-bimy)]2, in order to
compensate for the absence of halide in C·HBF4 (Scheme 2).
Complexes 1−4 were isolated as yellow solids in moderate to
good yields, which are soluble in most organic solvents, such as
CH2Cl2, CHCl3, MeOH, and CH3CN, with the exception of
nonpolar ones, such as hexane and diethyl ether. Their
formation is supported by ESI mass spectra, where a base
peak corresponding to the [M − Br]+ fragment and a smaller
signal assignable to [2M − Br]+ are present in all cases. In the
1H NMR spectra, the absence of downfield signals for the
synthesized by a “click reaction”16 involving a copper-catalyzed
[3 + 2] cycloaddition of azides and alkynes. Alkylation of these
1,2,3-triazoles with benzyl bromide or Meerwein’s salt14c gave
rise to 1,3-disubstituted triazolium salts (A·HBr−D·HBr) in mod-
erate to good yields, which served as ligand precursors for 1,2,3-
triazolin-5-ylidenes (Scheme 1). The yields for the alkylation
Scheme 2. Syntheses of Complexes 1−4
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i
triazolium salts and the presence of peaks from both Pr2-bimy
and the trz ligands also corroborate the formation of the
expected hetero-bis(carbene) complexes. The characteristic
multiplets for the C−H protons of the isopropyl groups of the
iPr2-bimy ligand range from 6.05 to 5.73 ppm, which are shifted
upfield in comparison to those of the precursor [PdBr2(iPr2-
bimy)]2 (cf. 6.54 ppm). Moreover, due to the loss of symmetry
upon complex formation, two C−H proton signals are observed
of the trz ligand, the determined order also correctly reflects
the increasing positive inductive effects of the groups Ph <
Bn < Pr. This result demonstrates that substituent effects
i
are indeed present and that they can be efficiently measured
using our new 13C NMR-based electronic parameter. It must be
highlighted that the lower sensitivity of common IR-based
methodologies is not sufficient to discern electronic influences
brought about by different substituents.14b,15b Notably, the
electronic properties of carbene ligands in complexes 2 and 3
that differ only in their N3 substituents cannot be further
separated even on our 13C NMR scale. Apparently, it is difficult
to discern remote and minute variations six bonds away from
the carbene probe.
Trifluoroacetato Complexes. Changing halido to trifluoro-
acetato ligands is a common practice to improve the solubilities
and catalytic activities of transition-metal complexes.19 Reac-
tions of complexes 1, 2, and 4 with 2 equiv of AgO2CCF3 gave
rise to new hetero-bis(carbene) complexes [Pd(O2CCF3)2(iPr2-
bimy)(trz)] (5−7) bearing labile trifluoroacetato ligands with
the precipitation of AgBr (Scheme 3).
1
in the H NMR spectrum of complex 1. One additional septet
appears at 6.05 ppm for complex 4, which is assigned to the
isopropyl group of trz ligands. For complexes 2 and 3, however,
only one multiplet integrating to 2 is observed in each spectrum
due to accidental overlap.
In the 13C NMR spectra of complexes 1−4, two carbene
signals are observed as expected. The downfield signals ranging
i
from 180.3 to 181.2 ppm are assigned to Pr2-bimy, whereas
those ranging from 157.9 to 160.6 ppm are attributed to the
1,2,3-triazole-derived carbenes. In comparison to the donor
strengths of other carbene ligands previously determined on the
13C NMR spectroscopic scale,11 1,2,3-triazolin-5-ylidenes prove
to be stronger donors than classical NHCs of types I−III
(Figure 1), which can be explained by a reduced electron-
withdrawing effect of only one nitrogen atom adjacent to the
carbene centers. Moreover, they are weaker than some non-
classical carbenes, such as pyrazolin-3-ylidenes IV and meso-
ionic imidazolin-4-ylidenes V (Figure 1), which is attributable
to the presence of a third electron-withdrawing nitrogen atom
in the triazole-derived systems. Furthermore, they show similar
donating abilities with indazolin-3-ylidenes VI.11
Compared with their precursors, the newly formed com-
plexes 5−7 are, in general, more soluble in common organic
solvents, which is a common feature of trifluoroacetato com-
plexes.19 All three compounds are mixtures of cis and trans
isomers, as evidenced by two sets of signals observed in their
1H NMR spectra. In their 13C NMR spectra, all the carbene
signals are shifted upfield by 4−5 ppm, due to the replacement
of bromido with less electron-donating trifluoroacetato ligands,
resulting in generally more Lewis acidic Pd centers. The signals
of other carbon atoms of triazole rings are shifted downfield by
0.7−1 ppm.
i
A closer inspection of the Pr2-bimy chemical shifts among
the four trz complexes 1−4 revealed increasing downfield shifts
in the order 1 (180.3 ppm) < 2/3 (180.8 ppm) < 4 (181.2 ppm),
reflecting a stepwise increase of the trz ligand’s donor ability.17
Because complexes 1, 2, and 4 differ only in the N1 substituent
Single crystals of 7 suitable for X-ray diffraction analysis were
grown from a concentrated CH2Cl2 solution as a representative.
Its molecular structure depicted in Figure 2 shows that both
Figure 1. Donor abilities of NHCs on the 13C NMR scale.18
Scheme 3. Syntheses of Complexes 5−7
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Table 1. Catalyst Screening of the Direct Arylation of
a
Pentafluorobenzene
b
entry
catalyst
catalyst loading (mol %)
yield (%)
TON
1
2
3
4
2
2
2
1
1
4
5
6
7
5
54
67
60
82
48
44
85
82
70
11
67
1
0.5
0.5
0.5
0.5
0.5
0.5
0.5
120
164
96
c
5
6
7
8
9
88
Figure 2. Molecular structure of 7 showing 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and
bond angles [deg]: Pd1−C1 2.025(5), Pd1−O1 2.055(4), Pd1−C14
2.031(5), Pd1−O3 2.028(4); C1−Pd1−O1 91.74(17), O1−Pd1−C14
90.36(18), C14−Pd1−O3 89.15(18), O3−Pd1−C1 88.84(18), C1−
Pd1−C14 175.6(2), O1−Pd1−O3 178.71(16).
170
164
140
a
Reaction conditions: precatalyst, K2CO3 (1.1 equiv), pentafluoro-
benzene (1.1 equiv), 4-bromotoluene (0.3 mmol), DMA (0.2 mL),
b
c
120 °C, 24 h. Isolated yield. 140 °C.
bromido ligands have been replaced with trifluoroacetato
ligands, which bind to the Pd center in a monodentate manner.
In addition, they are arranged trans to each other. The two
remaining coordination sites are taken by one benzimidazole-
and one triazole-derived carbene. The two Pd−Ccarbene bond
lengths [2.025(5) and 2.031(5) Å] are essentially the same.
The dihedral angles between the carbene planes and the
[PdC2O2] coordination plane are 64.6(1)° and 51.1(2)°, which
deviate substantially from the ideal 90°, probably due to the
steric congestion around the metal center. The angle between
the two carbene planes amounts to 13.4(2)°.
Catalysis. Although Pd−NHC complexes have been widely
used in C−C coupling reactions (vide supra), not much
attention has been paid to the direct arylation via direct C−H
bond functionalization. Although both reactions may lead to
biaryls, direct arylation is superior to Suzuki coupling in the
sense that no boronic acid waste will be produced. Direct
arylation of pentafluorobenzene with aryl halides has been
studied with Pd(OAc)2 and phosphine ligands.20 Because
NHCs are widely recognized as phosphine mimics, it is worth
studying and comparing the performance of Pd−NHC
complexes in such reactions.
A preliminary test was carried out with 4-bromotoluene and
1.1 equiv of pentafluorobenzene catalyzed by 5 mol %
precatalyst 2 in the presence of 1.1 equiv of K2CO3 in DMA
at 120 °C for 24 h, which resulted in a moderate 54% yield of
the desired product (Table 1, entry 1). Surprisingly, when the
catalyst loading was decreased to 1 mol %, the yield increased
to 67% (entry 2). It has been proven in the Heck- and Suzuki-
type reactions that a higher catalyst loading may result in a
decrease in the performance as more catalysts may decompose
and aggregate to Pd black, which hampers the catalytic pro-
cess.21 A 60% yield was still obtained when the loading was
further decreased to 0.5 mol % (entry 3). With 0.5 mol %
catalyst, precatalyst 1 was also tested at both 120 and 140 °C.
At 120 °C, complex 1 outperformed 2 as the yield increased to
82% (entry 4). At 140 °C, however, the yield dramatically
dropped to 48% (entry 5). Pd black was observed under this
condition, which shut down the catalysis to some extent.21
With the optimized reaction condition, precatalysts 4−7 were
screened (entries 6−9). It was found that the most electron-
rich complex 4 was not as efficient as 1 and 2, which shows that
complexes bearing less donating 1,2,3-triazolin-5-ylidenes are
better precatalysts for the direct arylation of pentafluoro-
benzene. The same trend was observed for the trifluoroacetato
complexes, as 5 proved to be a better performer than 6 and 7.
As expected, they are, in general, better precatalysts than their
bromido analogues. For instance, the reaction with 4 only gave
44% yield, while 70% yield was obtained when catalyzed by the
trifluoroacetato analogue 7. Similarly, 5 performed better than
1, and 6 outperformed 2. Overall, 5 gave rise to the best catalyst
for the direct arylation of pentafluorobenzene.
The scope of this reaction was studied with the best
precatalyst 5. The reaction proceeded well with aryl halide
bearing electron-donating groups. An 88% yield was obtained
with 4-bromoanisole (Table 2, entry 1). The results are also
good with 3-bromotoluene (80% yield, entry 2) and 3-
bromoanisole (95% yield, entry 3).
However, no conversion was detected with 2-bromotoluene
(entry 4). Apparently, the steric effect is the key factor in this
case. The yields also dropped substantially for the couplings
with electron-withdrawing aryl bromides (4-bromochloroben-
zene, 54%, entry 5; 4-bromobenzonitrile, 18%, entry 6). It is
noteworthy that, in the case of 4-bromochlorobenzene, no
doubly coupled product was detected at all, although 3 equiv of
pentafluorobenzene was added. In contrast, under the same
condition, the reaction with 1,3-dibromobenzene only gave the
doubly coupled product, although in a low yield (20%, entry 7).
CONCLUSION
■
We have reported a series of trans-[PdBr2(iPr2-bimy)(trz)]
complexes 1−4 bearing iPr2-bimy and mesoionic 1,2,3-triazolin-
5-ylidenes with different substituents. The study of their 13C
NMR spectra shows that 1,2,3-triazolin-5-ylidenes are, in general,
stronger donors than normal NHCs, while weaker than some
nonclassical NHCs, for example, pyrazolin-3-ylidenes and meso-
ionic imidazolin-4-ylidenes. Moreover, this study clearly shows
that the donor strength of 1,2,3-triazolin-5-ylidene decreases
with decreasing +I effect of the substituent at the N1 position.
Previously, such substituent effects on the donor ability of
triazolin-5-ylidenes could not be discerned by classical IR-based
methods, highlighting the superiority of our 13C NMR-based
methodology in detecting particularly fine differences among
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a
b
Table 2. Direct Arylation of Pentafluorobenzene Catalyzed by Complex 5
a
Reaction conditions: precatalyst (1 mol %), K2CO3 (1.1 equiv), pentafluorobenzene (1.5 equiv), aryl halide (0.3 mmol, 0.15 mmol in the case of
b
dihalide), DMA (0.2 mL), 120 °C, 24 h. Isolated yields.
was cooled to ambient temperature, diethyl ether (20 mL) was added
and the resulting suspension was filtered. The precipitate was washed
repetitively with copious diethyl ether until the starting materials were
ligands. The trifluoroacetato analogues 5−7 of complexes 1, 2,
and 4 have also been synthesized through salt metathesis with
AgO2CCF3. The catalytic activities of these six complexes were
compared in the direct arylation of pentafluorobenzene. Within
this series, complexes with less donating ligands perform better
in this catalysis and trifluoroacetato complexes outperformed
fully removed. The product was isolated as a white solid (50 mg,
1
0.127 mmol, 32%). H NMR (500 MHz, CDCl3): δ 10.18 (s, 1H,
NCH), 8.24 (d, 2H, Ar−H), 7.81 (m, 2H, Ar−H), 7.62−7.60 (m, 3H,
Ar−H), 7.52 (br-s, 3H, Ar−H), 7.37−7.34 (m, 3H, Ar−H), 7.17
(d, 2H, Ar−H), 5.91 (s, 2H, NCH2). 13C{1H} NMR (125.76 MHz,
their bromido analogues. Current research in our group is
ongoing to extend our donor strength study to a wider range of
ligands in an attempt to establish a unified electronic parameter
and to explore the catalytic applications of the respective trans-
[PdBr2(iPr2-bimy)L] complex probes.
CDCl3): 144.7 (s, NCPh), 135.4, 132.7, 132.6, 132.0, 131.1, 130.7,
130.4, 130.3, 130.1, 129.0, 128.8, 122.4, 122.2 (s, Ar−C), 56.8 (s, NCH2).
MS (ESI): m/z = 312 [M − Br]+.
B·HBr. A mixture of Bn-triazole (118 mg, 0.5 mmol) and benzyl-
bromide (1 mL) was heated in neat at 100 °C overnight. Diethyl ether
(20 mL) was added, and the resulting suspension was filtered. The
precipitate was washed repetitively with diethyl ether until all the
starting materials were removed. The compound was isolated as a
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all operations
were performed without taking precautions to exclude air and mois-
ture, and all solvents and chemicals were used as received. Ph-triazole,
1
white solid (93 mg, 0.23 mmol, 46%). H NMR (300 MHz, CDCl3):
δ 9.84 (s, 1H, NCH), 7.75−7.06 (m, 15H, Ar−H), 6.25, 5.74
(s, 2H, NCH2). 13C{1H} NMR (75.47 MHz, CDCl3): 143.6 (s,
NCPh), 132.7, 132.5, 132.1, 131.2, 131.0, 130.5, 130.4, 130.3, 130.1,
129.2, 122.5 (s, Ar−C), 58.4, 56.8 (s, NCH2). MS (ESI): m/z = 326
[M − Br]+.
14c
i
Bn-triazole, Pr-triazole, and salt C·HBF4
have been synthesized
according to reported procedures.22 1H, 13C, and 19F NMR spectra
were recorded on a Bruker ACF 300 spectrometer or AMX 500
spectrophotometer, and the chemical shifts (δ) were internally refer-
enced to the residual solvent signals relative to tetramethylsilane (1H, 13C)
or externally to CF3CO2H (19F NMR). ESI mass spectra were mea-
sured using a Finnigan MAT LCQ spectrometer. Elemental analyses
were performed on a PerkinElmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
i
D·HBr. Pr-triazole (94 mg, 0.5 mmol) and benzylbromide (1 mL)
were heated in neat at 100 °C overnight. After reaction, the mixture
was allowed to cool to ambient temperature. Diethyl ether (20 mL)
was added to the resulting light yellow solution, and some brown oil
came out. The mixture was decanted, and the brown oil was redis-
solved in CH2Cl2. The solution was added dropwise into diethyl ether
(10 mL), and the suspension was filtered. The precipitate was washed
A·HBr. Ph-triazole (86 mg, 0.4 mmol) and benzylbromide (3 mL)
were heated in neat at 100 °C for 3 days. After the reaction mixture
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repetitively with copious diethyl ether until the starting materials were
fully removed. The product was isolated as a brown solid (158 mg,
0.44 mmol, 88%). 1H NMR (500 MHz, CDCl3): δ 9.75 (s, 1H, NCH),
7.60−7.53 (m, 5H, Ar−H), 7.37−7.35 (m, 3H, Ar−H), 7.09 (d, 2H,
Ar−H), 5.71 (s, 2H, NCH2), 5.55 (m, 1H, 3J(H, H) = 7.1 Hz,
suspension was filtered through Celite, and the solvent of the filtrate
was removed. The crude product was purified by column
chromatography (SiO2, ethyl acetate/hexane, 3:7) to give the desired
compound as a yellow solid (60 mg, 0.08 mmol, 57%). 1H NMR (500
MHz, CDCl3): δ 7.85−7.83 (dd, 2H, Ar−H), 7.53−7.49 (m, 4H, Ar−
H), 7.47−7.45 (m, 1H, Ar−H), 7.33−7.31 (m, 3H, Ar−H), 7.15−7.13
(m, 2H, Ar−H), 7.07−7.05 (m, 2H, Ar−H), 6.05 (m, 1H, 3J(H, H) =
3
CH(CH3)2), 1.80 (d, 6H, J(H, H) = 7.1 Hz, CH(CH3)2). 13C{1H}
NMR (125.76 MHz, CDCl3): 143.6 (s, NCPh), 132.7, 132.2, 131.5,
130.5, 130.3, 130.1, 129.5, 128.6, 122.6 (s, Ar−C), 59.7 (s, NCH2),
56.1 (s, NCH), 23.1 (s, CH3). MS (ESI): m/z = 278 [M − Br]+.
trans-[PdBr2(iPr2-bimy)(A)](1). A mixture of salt A·HBr (50 mg,
0.13 mmol), [PdBr2(iPr2-bimy)]2 (60 mg, 0.06 mmol), and Ag2O
(18 mg, 0.08 mmol) was stirred in CH2Cl2 (15 mL) at ambient
temperature overnight shielded from light. The resulting suspension
was filtered over Celite, and the filtrate was removed in vacuo. The
residue was subjected to column chromatography (SiO2, ethyl acetate/
hexane, 3:7), and the pure compound was isolated as a yellow solid
3
7.1 Hz, CH(CH3)2), 5.90 (m, 1H, J(H, H) = 7.1 Hz, CH(CH3)2),
3
5.74 (m, 2H, J(H, H) = 7.1 Hz, CH(CH3)2), 5.44 (s, 2H, NCH2),
1.86 (d, 6H, 3J(H, H) = 7.1 Hz, CH(CH3)2), 1.82 (d, 6H, 3J(H, H) =
3
7.1 Hz, CH(CH3)2), 1.53 (d, 6H, J(H, H) = 7.1 Hz, CH(CH3)2).
13C{1H} NMR (125.76 MHz, CDCl3): 181.2 (s, NCN−iPr2-bimy),
157.9 (s, NCCPh), 144.9, 134.3, 134.2, 134.17, 131.7, 129.9, 129.7,
129.3, 129.2, 128.9, 128.1, 122.3, 113.0, 112.9 (s, Ar−C), 58.7, 54.1,
54.0, 53.7 (s, NCH2 + NCH), 23.5, 21.9, 21.3 (s, CH3). Anal. Calcd
for C31H37Br2N5Pd: C, 49.92; H, 5.00; N, 9.39. Found: C, 49.64; H,
4.95; N, 9.40%. MS (ESI): m/z = 666 [M − Br]+, 1412 [2M − Br]+.
[Pd(O2CCF3)2(iPr2-bimy)(A)](5). A mixture of 1 (51 mg, 0.065
mmol) and AgO2CCF3 (29 mg, 0.13 mmol) was suspended in
CH3CN (10 mL) and heated at 70 °C overnight shielded from light.
The reaction mixture was filtered over Celite, and the solvent of the
filtrate was evaporated off. The resulting yellow solid was redissolved
in CH2Cl2 (5 mL) and passed through silica gel. The solvent of the
filtrate was removed to give the product as a yellow solid (44 mg,
0.051 mmol, 80%). 1H NMR (500 MHz, CDCl3): δ 8.64 (d, 2H, Ar−
H), 8.01−7.99 (m, 2H, Ar−H), 7.68−7.63 (m, 3H, Ar−H), 7.59−7.57
(m, 3H, Ar−H), 7.52−7.44 (m, 2H, Ar−H), 7.36−7.31 (m, 3H, Ar−
H), 7.19−7.17 (m, 2H, Ar−H), 7.10−7.09 (m, 2H, Ar−H), 6.06−6.01
(m, 2H, 3J(H, H) = 7.1 Hz, CH(CH3)2), 5.56 (s, 2H, NCH2), 1.60 (d,
12H, 3J(H, H) = 7.1 Hz, CH(CH3)2). Signals of the minor isomer are
not assigned due to overlap. The ratio of the isomeric pair is around
4:1. 13C{1H} NMR (125.77 MHz, CDCl3): 176.5 (s, NCN−iPr2-
bimy), 162.2 (q, 2J(C, F) = 36.7 Hz, COO), 155.2 (s, NCCPh), 146.9,
140.0, 133.8, 133.7, 131.7, 130.9, 130.7, 130.6, 129.9, 129.7, 129.5,
129.4, 129.1, 128.8, 128.6, 128.1, 127.9, 125.9, 123.0 (s, Ar−C), 114.8
(q, 1J(C, F) = 283.2 Hz, CF3), 113.4 (s, Ar−C), 54.3, 54.0 (s, NCH2 +
NCH), 22.0, 21.9 (s, CH3). 19F NMR (282.37 MHz, CDCl3): 1.98 (s,
CF3). Anal. Calcd for C38H35F6N5O4Pd: C, 53.94; H, 4.17; N, 8.28.
Found: C, 53.71; H, 4.14; N, 8.62%. MS (ESI): m/z = 733 [M −
OOCCF3]+.
1
(44 mg, 0.056 mmol, 44%). H NMR (300 MHz, CDCl3): δ 8.57−
8.53 (dd, 2H, Ar−H), 7.99−7.96 (dd, 2H, Ar−H), 7.63−7.56 (m, 6H,
Ar−H), 7.47−7.42 (m, 2H, Ar−H), 7.36−7.34 (m, 3H, Ar−H), 7.20−
3
7.10 (m, 4H, Ar−H), 5.90 (m, 1H, J(H, H) = 7.1 Hz, CH(CH3)2),
3
5.73 (m, 1H, J(H, H) = 7.1 Hz, CH(CH3)2), 5.55 (s, 2H, NCH2),
1.64 (d, 6H, 3J(H, H) = 7.1 Hz, CH(CH3)2), 1.53 (d, 6H, 3J(H, H) =
7.1 Hz, CH(CH3)2). 13C{1H} NMR (75.47 MHz, CDCl3): 180.3
(s, NCN−iPr2-bimy), 160.6 (s, NCCPh), 145.9, 140.6, 134.3, 134.2,
133.7, 132.0, 130.4, 130.3, 129.8, 129.7, 129.4, 129.1, 128.8, 128.6,
126.1, 122.3, 113.0 (s, Ar−C), 54.4, 54.2, 54.0 (s, NCH2 + NCH),
21.6, 21.5 (s, CH3). Anal. Calcd for C34H35Br2N5Pd: C, 52.36; H, 4.52;
N, 8.98. Found: C, 52.13; H, 4.61; N, 9.19%. MS (ESI): m/z = 700
[M − Br]+, 1480 [2M − Br]+.
trans-[PdBr2(iPr2-bimy)(B)](2). A mixture of salt B·HBr (93 mg,
0.23 mmol), [PdBr2(iPr2-bimy)]2 (107 mg, 0.11 mmol), and Ag2O
(28 mg, 0.12 mmol) was stirred in CH2Cl2 (15 mL) shielded from
light. The suspension was filtered over Celite, and the solvent of the
filtrate was removed to give the product as a yellow solid (159 mg,
1
0.20 mmol, 87%). H NMR (500 MHz, CDCl3): δ 7.85 (s, 2H, Ar−
H), 7.74−7.73 (m, 2H, Ar−H), 7.52−7.39 (m, 9H, Ar−H), 7.30 (br-s,
2H, Ar−H), 7.14 (s, 2H, Ar−H), 7.03 (s, 2H, Ar−H), 6.16 (s, 2H,
NCH2), 5.96 (m, 2H, CH(CH3)2), 5.42 (s, 2H, NCH2), 1.71 (br-s,
6H, CH(CH3)2), 1.57 (br-s, 6H, CH(CH3)2). 13C{1H} NMR (125.76
MHz, CDCl3): 180.8 (s, NCN−iPr2-bimy), 160.4 (s, NCCPh), 146.0,
135.4, 134.3, 133.9, 131.8, 130.1, 129.7, 129.4, 129.35, 129.0, 128.9,
128.2, 122.3, 113.0 (s, Ar−C), 59.0, 54.2, 54.0 (s, NCH2 + NCH),
21.7, 21.4 (s, CH3). Anal. Calcd for C35H37Br2N5Pd: C, 52.95; H, 4.70;
N, 8.82. Found: C, 52.54; H, 4.51; N, 8.86%. MS (ESI): m/z = 714
[M − Br]+, 1508 [2M − Br]+.
[Pd(O2CCF3)2(iPr2-bimy)(B)](6). This complex was synthesized in
analogy to complex 5 from complex 2 (complex 2: 50 mg, 0.063
mmol; AgOOCCF3: 28 mg, 0.126 mmol; Yield: 32 mg, 0.037 mmol,
1
59%). H NMR (500 MHz, CDCl3): δ 7.85−7.83 (dd, 2H, Ar−H),
7.71 (d, 2H, Ar−H), 7.56−7.52 (m, 5H, Ar−H), 7.47−7.41 (m, 2H,
Ar−H), 7.31−7.27 (m, 4H, Ar−H), 7.20−7.18 (dd, 2H, Ar−H),
6.94−6.92 (m, 2H, Ar−H), 6.14−6.10 (m, 4H, NCH2 + CH(CH3)2),
5.44 (s, 2H, NCH2), 1.66 (d, 12H, CH(CH3)2). Signals of the minor
isomer are not assigned due to overlap. The ratio of the isomeric pair
is around 6:1. 13C{1H} NMR (125.76 MHz, CDCl3): 176.3 (s,
trans-[PdBr2(iPr2-bimy)(C)](3). A mixture of [PdBr2(iPr2-bimy)]2
(94 mg, 0.1 mmol) and tetrabutylammonium bromide (64 mg,
0.2 mmol) was heated in CHCl3 (5 mL) at refluxing temperature for
3 h. All the volatiles were removed in vacuo to give an orange solid
that was redissolved in CH2Cl2 (15 mL). Salt C·HBF4 (67 mg,
0.2 mmol) and Ag2O (28 mg, 0.12 mmol) were added to the solution,
and the resulting mixture was stirred at ambient temperature overnight
shielded from light. The suspension was filtered through Celite. The
filtrate was extracted with H2O (3 × 10 mL) and dried over Na2SO4.
All the volatiles were removed to give the product as a yellow solid
2
NCN−iPr2-bimy), 162.0 (q, J(C, F) = 35.8 Hz, COO), 155.5 (s,
NCCPh), 146.7, 135.3, 134.0, 133.7, 131.4, 130.5, 129.8, 129.7, 129.6,
129.5, 129.3, 129.2, 128.1, 127.8, 123.3, 123.0 (s, Ar−C), 114.9 (q,
1J(C, F) = 290.5 Hz, CF3), 113.7, 113.2 (s, Ar−C), 58.8, 54.5, 54.2,
54.0 (s, NCH2 + NCH), 22.1, 21.9 (s, CH3). 19F NMR (282.37 MHz,
CDCl3): 2.15 (s, CF3). Anal. Calcd for C39H37F6N5O4Pd: C, 54.46; H,
4.34; N, 8.14. Found: C, 54.26; H, 4.36; N, 8.31%. MS (ESI): m/z =
787 [M − O2CCF3 + CH3CN]+.
1
(129 mg, 0.18 mmol, 90%). H NMR (500 MHz, CDCl3): δ 8.01
(d, 2H, Ar−H), 7.79 (d, 2H, Ar−H), 7.56−7.39 (m, 8H, Ar−H),
3
7.16−7.14 (m, 2H, Ar−H), 6.13 (s, 2H, NCH2), 6.05 (m, 2H, J(H,
H) = 7.1 Hz, CH(CH3)2), 1.73 (d, 6H, 3J(H, H) = 7.1 Hz,
[Pd(O2CCF3)2(iPr2-bimy)(D)](7). This complex was synthesized in
analogy to complex 5 from complex 4 (complex 4: 50 mg, 0.067
mmol; AgOOCCF3: 30 mg, 0.134 mmol; Yield: 45 mg, 0.055 mmol,
3
CH(CH3)2), 1.63 (d, 6H, J(H, H) = 7.1 Hz, CH(CH3)2). 13C{1H}
NMR (125.76 MHz, CDCl3): 180.8 (s, NCN−iPr2-bimy), 160.0
(s, NCCPh), 145.8, 135.2, 134.2, 131.3, 130.0, 129.9, 129.4, 129.1,
128.9, 128.85, 122.3, 113.0 (s, Ar−C), 59.0, 54.2, 54.0 (s, NCH2 +
NCH), 37.6 (s, NMe3), 21.6, 21.4 (s, CH3). Anal. Calcd for
C29H33Br2N5Pd: C, 48.52; H, 4.63; N, 9.76. Found: C, 48.03; H, 4.51;
N, 9.84%. MS (ESI): m/z = 638 [M − Br]+, 1356 [2M − Br]+.
trans-[PdBr2(iPr2-bimy)(D)](4). A mixture of salt D·HBr (51 mg,
0.14 mmol), [PdBr2(iPr2-bimy)]2 (67 mg, 0.07 mmol), and Ag2O
(20 mg, 0.09 mmol) was suspended in CH2Cl2 (15 mL) and stirred at
ambient temperature overnight shielded from light. The resulting
1
83%). H NMR (500 MHz, CDCl3): δ 7.87−7.85 (dd, 2H, Ar−H),
7.55−7.52 (m, 5H, Ar−H), 7.33−7.32 (m, 3H, Ar−H), 7.21−7.19 (m,
3
2H, Ar−H), 6.97−6.95 (m, 2H, Ar−H), 6.16 (m, 2H, J(H, H) =
3
7.1 Hz, CH(CH3)2), 5.82 (m, 1H, J(H, H) = 7.1 Hz, CH(CH3)2),
5.29 (s, 2H, NCH2), 1.83 (d, 6H, 3J(H, H) = 7.1 Hz, CH(CH3)2), 1.69
3
(d, 6H, J(H, H) = 7.1 Hz, CH(CH3)2). Signals of the minor isomer
are not assigned due to overlap. The ratio of the isomeric pair is around
5:1. 13C{1H} NMR (125.76 MHz, CDCl3): 177.0 (s, NCN−iPr2-
bimy), 162.0 (q, 2J(C, F) = 35.7 Hz, COO), 153.1 (s, NCCPh), 145.8,
410
dx.doi.org/10.1021/om2010029 | Organometallics 2012, 31, 405−412

Organometallics
Article
134.4, 133.8, 131.4, 130.3, 129.8, 129.76, 129.4, 129.1, 128.3, 127.5,
123.3, 123.0 (s, Ar−C), 114.9 (q, J(C, F) = 294.2 Hz, CF3), 113.4
(s, Ar−C), 58.6, 54.5, 54.1, 53.9 (s, NCH2 + NCH), 23.6, 22.0, 21.99
(s, CH3). 19F NMR (282.37 MHz, CDCl3): 2.08 (s, CF3). Anal. Calcd
for C35H37F6N5O4Pd: C, 51.76; H, 4.59; N, 8.62. Found: C, 51.52; H,
4.74; N, 8.71%. MS (ESI): m/z = 699 [M − O2CCF3]+.
Organometallics 2008, 27, 202. (b) Leuthausser, S.; Schwarz, D.;
̈
1
Plenio, H. Chem.Eur. J. 2007, 13, 7195. (c) Herrmann, W. A.;
Schutz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25, 2437.
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(d) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.;
Crabtree, R. H. Organometallics 2004, 23, 2461.
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Chem. Soc. 2006, 128, 16514. (b) Khramov, D. M.; Lynch, V. M.;
Bielawski, C. W. Organometallics 2007, 26, 6042. (c) Khramov, D. M.;
Rosen, E. L.; Er, J. A. V.; Vu, P. D.; Lynch, V. M.; Bielawski, C. W.
Tetrahedron 2008, 64, 6853.
Direct Arylation of Pentafluorobenzene. In a typical run, a
Schlenk tube was charged with precatalyst, K2CO3 (0.33 mmol), and
aryl halide if it is a solid. The reaction vessel was evacuated and refilled
with nitrogen three times. Aryl halide (if it is a liquid), penta-
fluorobenzene (0.3 mmol), and DMA (0.2 mL) were added, and the
reaction was placed in a preheated oil bath and stirred. After 24 h, the
mixture was cooled to the ambient temperature, and dichloromethane
(2 mL) was added. The suspension was filtered through Celite, and
the residue was washed with dichloromethane (2 × 2 mL). The
solvent of the filtrate was removed, and the crude product was loaded
onto silica gel using hexane or a hexane/ether mixture as an eluent to
get pure compound.
X-ray Diffraction Studies. X-ray data for 7 were collected with a
Bruker AXS SMART APEX diffractometer, using Mo Kα radiation at
100(2) K with the SMART suite of Programs.23 Data were processed
and corrected for Lorentz and polarization effects with SAINT,24 and
for absorption effects with SADABS.25 Structural solution and
refinement were carried out with the SHELXTL suite of programs.26
The structure was solved by direct methods to locate the heavy atoms,
followed by difference maps for the light, non-hydrogen atoms. All
non-hydrogen atoms were generally given anisotropic displacement
parameters in the final model. All H atoms were put at calculated
positions. A summary of the most important crystallographic data is
given in ref 27.
(9) Kuchenbeiser, G.; Soleilhavoup, M.; Donnadieu, B.; Bertrand, G.
Chem.Asian J. 2009, 4, 1745.
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2009, 28, 5395.
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(14) (a) Lalrempuia, R.; McDaniel, N. D.; Muller-Bunz, H.;
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Bernhard, S.; Albrecht, M. Angew. Chem., Int. Ed. 2010, 49, 9765.
(b) Poulain, A.; Canseco-Gonzalez, D.; Hynes-Roche, R.; Muller-
̈
Bunz, H.; Schuster, O.; Stoeckli-Evans, H.; Neels, A.; Albrecht, M.
Organometallics 2011, 30, 1021. (c) Kilpin, K. J.; Paul, U. S. D.; Lee,
A.; Crowley, J. D. Chem. Commun. 2011, 47, 328. (d) Saravanakumar,
R.; Ramkumar, V.; Sankararaman, S. Organometallics 2011, 30, 1689.
(e) Prades, A.; Peris, E.; Albrecht, M. Organometallics 2011, 30, 1162.
(f) Schuster, E. M.; Botoshansky, M.; Gandelman, M. Dalton Trans.
2011, 40, 8764. (g) Karthikeyan, T.; Sankararaman, S. Tetrahedron
Lett. 2009, 50, 5834. (h) Cai, J.; Yang, X.; Arumugam, K.; Bielawski,
C. W.; Sessler, J. L. Organometallics 2011, 30, 5033. (i) Inomata, S.;
Hiroki, H.; Terashima, T.; Ogata, K.; Fukuzawa, S. Tetrahedron 2011,
67, 7263. (j) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.;
Bertrand, G. Organometallics 2011, 30, 6017. (k) Bernet, L.;
Lalrempuia, R.; Ghattas, W.; Mueller-Bunz, H.; Vigara, L.; Llobet,
A.; Albrecht, M. Chem. Commun. 2011, 47, 8058. (l) Hohloch, S.; Su,
C.-Y.; Sarkar, B. Eur. J. Inorg. Chem. 2011, 3067. (m) Schulze, B.;
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for 7 as a CIF file and H and 13C NMR
1
spectra of complexes 1−7. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chmhhv@nus.edu.sg.
■
ACKNOWLEDGMENTS
■
The authors thank the National University of Singapore for
financial support (WBS R-143-000-410-112). Technical support
from staff at the CMMAC of our department is appreciated. In
particular, we thank Ms. Geok Kheng Tan and Prof. Lip Lin
Koh for determining the X-ray molecular structures.
Escudero, D.; Friebe, C.; Siebert, R.; Gorls, H.; Kohn, U.; Altuntas, E.;
̈
̈
Baumgaertel, A.; Hager, M. D.; Winter, A.; Dietzek, B.; Popp, J.;
Gonzal
́
ez, L.; Schubert, U. S. Chem.Eur. J. 2011, 17, 5494. (n) Keitz,
B. K.; Bouffard, J.; Bertrand, G.; Grubbs, R. H. J. Am. Chem. Soc. 2011,
133, 8498. (o) Nakamura, T.; Terashima, T.; Ogata, K.; Fukuzawa, S.-i.
Org. Lett. 2011, 13, 620. (p) Nakamura, T.; Ogata, K.; Fukuzawa, S.-i.
Chem. Lett. 2010, 39, 920.
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(23) SMART, version 5.628; Bruker AXS Inc.: Madison, WI, 2001.
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(27) Formula, C35H37F6N5O4Pd; fw, 812.10; color, yellow; habit,
block; cryst size [mm], 0.26 × 0.20 × 0.12; temp [K], 100(2); cryst
syst, monoclinic; space group, P2(1)/c; a [Å], 13.266(5); b [Å],
13.365(5); c [Å], 20.934(8); α [deg], 90; β [deg], 102.354(12); γ
[deg], 90; V [Å3], 3626(2); Z, 4; Dc [g cm−3], 1.488; radiation used,
Mo Kα; μ [mm−1], 0.587; θ range [deg], 1.82−25.00; no. of unique
data, 6381; max., min transm, 0.9329, 0.8623; final R indices [I >
2σ(I)], R1 = 0.0612, wR2 = 0.1384; R indices (all data), R1 = 0.0842,
wR2 = 0.1482; goodness-of-fit on F2, 1.064; peak/hole [e Å−3], 2.272/
−0.691.
412
dx.doi.org/10.1021/om2010029 | Organometallics 2012, 31, 405−412