Synthesis of pincer-type N-heterocyclic carbene palladium complexes with a hemilabile ligand and their application in cross-coupling catalysis

Article abstract of DOI:10.1016/j.jorganchem.2014.05.022

Benzimidazolium salts containing both a neutral imine and a masked carboxylate functional group for potential metal chelation were prepared. Palladation of the ester-protected ligand afforded a N,C-bidentate carbene complex 4. Subsequent ester hydrolysis preserved the bidentate coordination mode and yielded complex 5 with a pending COOH group exclusively. However, when ester deprotection was carried out prior to metalation, the N,C,O-tridentate pincer-type coordinated palladium complex 7 was obtained. Proton-abstraction of the dangling COOH group in the bidentate ligand of complex 5 by treatment with a base led to the formation of the N,C,O-tridentate coordinated Pd system 7, and inversely, exposure of the tridentate bound Pd complex 7 with acid afforded the N,C-bidentate ligand coordination mode in complex 5, demonstrating hemilability of the oxygen donor site in the pincer ligand. All three palladium(II) complexes 4, 5, and 7 were evaluated in cross-coupling catalysis and revealed distinct activity differences that are dependent on the type of coupling (Suzuki vs. Heck) and the substrate (Ar-Br vs. Ar-Cl). These differences suggest that judicious choice of donor groups in pincer-type complexes is a viable strategy for catalyst optimization.

Full text of DOI:10.1016/j.jorganchem.2014.05.022

Accepted Manuscript
Synthesis of Pincer-type N-Heterocyclic Carbene Palladium Complexes with a
Hemilabile Ligand and their Application in Cross-Coupling Catalysis
Vivienne Leigh , Helge Mueller-Bunz , Martin Albrecht
PII:
S0022-328X(14)00259-9
10.1016/j.jorganchem.2014.05.022
JOM 18594
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 April 2014
Revised Date: 20 May 2014
Accepted Date: 21 May 2014
Please cite this article as: V. Leigh, H. Mueller-Bunz, M. Albrecht, Synthesis of Pincer-type N-
Heterocyclic Carbene Palladium Complexes with a Hemilabile Ligand and their Application
in Cross-Coupling Catalysis, Journal of Organometallic Chemistry (2014), doi: 10.1016/
j.jorganchem.2014.05.022.
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ACCEPTED MANUSCRIPT
Synthesis of Pincer-type N-Heterocyclic Carbene Palladium Complexes
with a Hemilabile Ligand and their Application in Cross-Coupling
Catalysis
Vivienne Leigh, Helge Mueller-Bunz, Martin Albrecht*
School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4,
Ireland
Fax: +35317162501; E-mail: martin.albrecht@ucd.ie
Abstract
Benzimidazolium salts containing both a neutral imine and a masked carboxylate functional
group for potential metal chelation were prepared. Palladation of the ester-protected ligand
afforded a N,C-bidentate carbene complex 4. Subsequent ester hydrolysis preserved the
bidentate coordination mode and yielded complex 5 with a pending COOH group exclusively.
However, when ester deprotection was carried out prior to metalation, the N,C,O-tridentate
pincer-type coordinated palladium complex 7 was obtained. Proton-abstraction of the
dangling COOH group in the bidentate ligand of complex 5 by treatment with a base led to
the formation of the N,C,O-tridentate coordinated Pd system 7, and inversely, exposure of the
tridentate bound Pd complex 7 with acid afforded the N,C-bidentate ligand coordination mode
in complex 5, demonstrating hemilability of the oxygen donor site in the pincer ligand. All
three palladium(II) complexes 4, 5, and 7 were evaluated in cross-coupling catalysis and
revealed distinct activity differences that are dependent on the type of coupling (Suzuki vs.
Heck) and the substrate (Ar–Br vs. Ar–Cl). These differences suggest that judicious choice of
donor groups in pincer-type complexes is a viable strategy for catalyst optimization.
O
N
N
OH
N
N
O
base
acid
N
N
N
X
Pd
O
N
Pd
X
X
active Suzuki catalytst with ArBr and ArCl
inactive Heck catalyst
active Suzuki catalytst with ArCl only
active Heck catalyst
1

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1. Introduction
Organometallic pincer-type complexes featuring a M–C bond that is supported by
different sets of chelating donor groups have found widespread application in catalysis [1].
Polydentate ligation enhances the stability of the metal-carbon bond and thus prevents metal
leaching. In addition to the incorporation of classic N, P, S, and O-donor groups for chelation,
N-heterocyclic carbenes (NHCs) [2] have become increasingly popular in the design of such
pincer-type ligands over the last decade [3], in particular due to their exceptional success in
transition metal catalysis [4] and beyond [5]. As strong σ donors that bind tightly to a broad
range of (late) transition metal centers, NHCs are generally [6] useful spectator ligands to
prevent complex decomposition [4]. The benefit of combining NHC ligands in a pincer ligand
scaffold for metal coordination has been exploited with various types of NHC systems
including for example imidazolylidenes [7], benzimidazolylidenes [8], pyridylidenes [9], and
triazolylidenes [10].
While originally, pincer ligands were C2-symmetric and featured two identical donor
sites in mutual trans position [11], recent work has increasingly focused on dissymmetric
tridentate systems that combine ligating groups with different donor characteristics to support
the M–C bond [12]. While neutral, soft [13] donors stabilize low-valent metal centers,
anionic, hard donors in contrast enhance the stability of high-valent and electron-poor metals,
and such bonding principles have recently started to emerge also with NHC complexes [14].
The design of ligands that possess both neutral and anionic donor sites may thus be of great
benefit in catalytic applications for stabilizing critical transition states and for enabling
concerted reaction pathways. Specifically, such a ligand design is expected to be
advantageous for catalytic cycles that involve redox processes which require the accessibility
of both high- and low-valent metal centers. Cross-coupling catalysis provides an attractive
probe for this concept since stabilization of Pd0 and PdII oxidation states is required for the
Ar–X activation and the C–C bond forming steps, respectively [15]. While a few examples of
NHC palladium complexes are known that contain both neutral and anionic donors (Figure 1)
[16], only one of these complexes was investigated in catalysis. Complex C with a weakly
coordinated sulfone ligand showed good activity in Suzuki-Miyaura cross-coupling reactions
using aryl bromides as substrates [16c].
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+
O
S
tBu
O
N
N
N
Br
O
Pd
tBu
O
Pd
N
O
H
I
Ni
N
N
Ph
N
N
NCMe
N
A
B
C
Figure 1 Pincer-type NHC complexes containing both anionic and neutral donors
Obviously, the concept has potential limitations. For example, if an already relatively stable
intermediate is further stabilized, the catalytic turnover frequency will decrease and catalyst
deactivation or even side reactions and irreversible catalyst decomposition pathways may
become predominant. Cross-coupling reactions with different substrate-dependent, rate-
determining steps are expected to provide insight into such decomposition pathways. Herein
we report the synthesis of benzimidazolylidene palladium complexes with an ancillary neutral
imine donor and a potentially chelating oxygen donor and their performance in Suzuki-
Miyaura and Heck-type arylation of haloarenes and olefins, respectively.
2. Results and discussion
2.1 Synthesis and modification of ligand coordination modes
The synthetic route to the preparation of the benzimidazolium ligand is shown in
Scheme 1. N-methyl-2-chloromethyl benzimidazole 1 was prepared via standard procedures
[17], and reacted with benzimidazole in the presence of KOH to form 2 as a white crystalline
1
solid. The formation of 2 was confirmed in the H NMR spectrum by the emergence of a
singlet at 7.99 ppm for the NCHN proton. Also, the presence of four new aromatic protons of
equal integration for the new benzimidazole supported formation of 2. The singlet for the CH2
group shifted significantly downfield from 4.85 ppm in 1 to 5.65 ppm in 2. Compound 2 was
then reacted with t-butyl bromoacetate to give the azolium salt 3. Upon formation of 3, the
compound was no longer soluble in CDCl3. Formation of the azolium salt was indicated in the
1H NMR spectrum by the new lowfield signal at 9.90 ppm for the acidic NCHN proton.
Moreover a new singlet emerged at 1.49 ppm with correct integration for the nine protons of
the t-butyl group.
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O
benzimidazole
KOH
O
Br
Cl
N
N
OtBu
N
N
O
N
N+
Br–
N
N
MeCN
MeCN
N
N
1
2
3
PdCl2
NaOAc
O
N
N
O
N
N
Pd
X
X
4
(X = Cl, Br)
Scheme 1 Synthesis of NHC palladium complex 4
Complexation of 3 was initially attempted using Ag2O and PdCl2 in an in-situ
transmetalation procedure. However, this reaction yielded mixtures of a monodentate and a
bidentate complex as indicated by 1H NMR spectroscopy and mass spectrometry. Metalation
was then carried out using PdCl2 and NaOAc in CH3CN, which afforded the N,C-bidentate
chelate complex 4 (Scheme 2). The product was identified by the loss of the acidic singlet at
1
δH 9.90 ppm in the H NMR spectrum, suggesting carbene coordination. The methyl groups
of the BOC group shift from δH 1.49 ppm to 1.37 ppm upon palladation. Furthermore, all
aromatic protons in 4 are slightly deshielded as compared to 3. The CH2 spacer groups appear
as two sets of AB doublets at δH 6.37 and 6.08 ppm (2JHH = 16.7 Hz), and at δH 5.79 and 5.58
ppm (2JHH = 17.3 Hz). The different chemical environment of each methylene proton
indicates chelation of the NHC ligand and/or rigid orientation of the NHC substituents. In the
13C NMR spectrum the carbene carbon was detected at δC 166.6 ppm, which is typical of
palladium-benzimidazolylidene complexes [8,18]. Upon complexation, the carbonyl carbon
shifts significantly downfield from δC = 165.5 ppm to 176.0 ppm.
Unambiguous evidence for the formation of complex 4 was obtained by single crystal
X-ray diffraction. Suitable crystals were obtained by diffusion of Et2O into a CHCl3 solution
of the complex. Selected bond lengths and angles are compiled in Table 1. The molecular
structure (Figure 2) features a square-planar palladium center with a N,C-bidentate chelating
benzimidazole-benzimidazolylidene ligand. The Pd–C bond length is 1.9760(19), which is
slightly longer than in related Pd-benzimidazolylidene complexes [19]. The complex
crystallized with a 5:3 ratio of chlorides and bromides coordinated to the palladium center due
to the presence of a Br– counterion in the ligand and Cl– ions in the metal precursor. From the
5

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X-ray structure, the COOtBu arm is in proximity to one of the halides bound to the metal
center. Due to chelation of the NHC ligand, rotation about the N(4)-CH2 bond is presumably
1
limited. This steric restriction may rationalize the splitting of the H NMR resonance of this
CH2 group into a AB doublet [20]. Accordingly, the AB pattern of a methylene signal is not
per se evidence for chelation. Further evidence that the AB doublet is not associated with
metal bonding was obtained from analysis of the carbonyl stretch vibration by IR
spectroscopy. A band at 1740 cm–1 was observed both for the ligand precursor 3 and in the
palladium complex 4, indicating no significant perturbation of the ester group as would be
expected from ester coordination.
Figure 2 Ortep representation of 4 (50% probability level, hydrogen atoms omitted for clarity).
Table 1 Selected bond lengths (Å) and angles (°) in complex 4 a)
Pd-C10
Pd-N1
Pd-X1
Pd-X2
1.9760(19)
2.0471(16)
2.3910(3)
2.4360(4)
C10-Pd-N1
C10-Pd-X1
N1-Pd-X1
C10-Pd-X2
N1-Pd-X2
X1-Pd-X2
86.72(7)
91.48(5)
170.55(5)
178.37(5)
94.29(5)
87.32(12)
a) X1: 54% Cl, 46% Br; X2: 71% Cl, 29% Br
Deprotection of complex 4 with trifluoroacetic acid (TFA) resulted in the clean cleavage
of the BOC group with no impact on the coordination of the ligand (Scheme 2). Complex 5
1
features only marginal shifts in the H NMR spectrum compared to 4. The most diagnostic
change was the loss of the singlet at 1.37 ppm for the t-butyl group and a downfield shift of
6

ACCEPTED MANUSCRIPT
one of the CH2 AB signals (δH 6.19 and 5.96 ppm, 2JHH = 16.7 Hz). Since the higher field AB
pattern is essentially unaffected, we tentatively attribute this latter signal to the CH2 group
interlinking the two benzimidazole fragments, and the former to the carbonyl-bound CH2
entity. In the 13C NMR spectrum of complex 5, the carbene carbon resonance was observed at
δC 168.6 ppm, slightly upfield as compared with the ester-functionalized carbene in complex
4. While the carbonyl carbon signal appears at δH 174.0 ppm and hence almost the same
frequency as in complex 4, the shift of the CO band from νCO 1740 to 1678 cm–1 in the IR
spectrum provides strong support for the formation of the carboxylic acid. According to
microanalysis, the two anionic ligands are Cl– and CF3COO–, the latter resulting from TFA-
mediated BOC group cleavage.
O
O
N
N
N
OH
N
O
TFA
N
N
X
X
N
Pd
X
Pd
X
N
CH2Cl2
4
(X = Cl, Br)
5 (X = Cl, CF3CO2)
base acid
O
N+
Br–
N
N
N
OH
TFA
PdCl2
NaOAc
O
3
N
N
CH2Cl2
Pd
N
O
N
X
6
7
Scheme 2 Synthesis of complexes 5 and 7 containing a pending or coordinated carboylate group, respectively.
Acid deprotection was also prerformed from the ligand precursor 3 prior to palladation,
thus affording compound 6 in moderate yields. Noteworthy, the deprotection of the ligand
proceeds in lower yields than the deprotection of the complex (48% vs. 78%). Upon
palladation starting from the deprotected ligand 6, the tridentate coordinated Pd complex 7
was obtained exclusively. Complex 7 was also prepared via transmetalation using Ag2O and
PdCl2 as the transmetalating agent. Two equivalents of Ag2O were needed to form the
complex, whereas no metalation was observed when only one equivalent was used. This
outcome is presumably due to the presence of two competing acidic sites for deprotonation,
7

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1
i.e. the carboxylic acid and the azolium proton. Complex 7 was characterized by H NMR
spectroscopy, which revealed the loss of the benzimidazolium proton at δH 9.92 ppm. The
1
most noteworthy features in the H NMR spectrum are the two singlets at δH 6.08 ppm and
5.08 ppm attributed to the two methylene linkers in complex 7. The singlet multiplicity
suggests either a molecular CS symmetry due to completely planar coordination of the pincer
ligand, or more likely for this type of ligands [21], fast puckering that involves wagging of the
benzimidazolylidene ligand with respect to the C–Pd–X axis. Rapid interconversion of the
two limiting conformations transforms pseudo-equatorial protons of the CH2 linkers into
pseudo-axial protons and vice versa. In agreement with such a puckered conformation, these
two singlets are significantly broadened (w1/2 = 5 and 6 Hz) as compared with the resonances
due to those groups in the ligand precursor 6 (w1/2 = 2 Hz), which may indicate dynamic
processes upon complexation. The methyl singlet shifts downfield from δH 3.95 ppm in 6 to
4.00 ppm in the complex. The carbene carbon was observed at δC 156.3 ppm, which is
considerably higher field than in complex 4 (δC 166.6 ppm). The carbonyl fragment is only
marginally affected by the coordination of the carboxylate and shifts from δC 168.2 ppm in 6
to 168.6 ppm in 7. This resonance is also at significantly higher field than that of complex 4
(δC 176.0 ppm). In IR spectroscopy, a broad signal at 1624 cm–1 was attributed to the C=O
vibration and a sharper band at 1385 cm–1 for the C–O bond vibration, which is in agreement
with reported values of coordinated carboxylates [22].
Coordination and dissociation of the O-donor in the pincer complexes 5 and 7 is pH-
dependent. The addition of 1 equiv. of HCl (1 M solution) to complex 7 afforded the
bidentate coordinated Pd complex 5 cleanly without further purification. Complex 5 was also
formed with 1 equiv. HBr (1 M solution), however, other acids such as H2SO4 and
CH3COOH had no effect on 7. This reactivity pattern is presumably due to the need of a
strongly coordinating anion to displace the carboxylate donor group in combination with the
stability of the chelate. Complex 7 was stable towards base. One equiv. NaOH (1 M) had no
apparent effect on the complex, and only after the introduction of a large exess (>10 equiv.)
decomposition started to be observed. The addition of 1 equiv. base such as NaOH, NaOAc
and NEt3 to the N,C-bidentately bound Pd complex 5 induced coordination of the carboxylate
group and formation of the tridentate chelate 7. These reactions were also scaled up and have
been carried out on a 500 mg scale and demonstrate a flexible behavior of the pincer ligand
and hemilabile coordination.
8

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2.2 Cross-coupling catalysis
The palladium(II) complexes 4, 5 and 7 were tested in the Suzuki-Miyaura coupling of
deactivated aryl bromides (bromoanisole) and activated aryl chlorides (chlorobenzaldehyde)
with phenylboronic acid at mild temperatures (50 °C, Table 2) [23]. Low temperatures were
chosen to prevent the formation of colloidal particles as much as possible [24]. Under these
conditions, the bidentate coordinated Pd complex 5 shows the highest activity with moderate
to good conversions (Table 2, entries 2 and 5). Complex 7 with a tridentate ligand and the
bidentate chelate 4 show both only low conversions for the coupling of 3-bromoanisole and
phenylboronic acid (entries 1 and 3). Although weak bases are sufficient to transform 5 into 7,
such a reaction does not seem to be relevant under catalytic conditions when considering the
distinct activities of these two complexes. Presumably, the preferred reaction of the base
involves the phenylboronic acid rather than the carboxylic acid in 5, thus inducing cross-
coupling catalysis. Remarkably, superior activity of 5 over 7 was only observed for aryl
bromide coupling. With aryl chlorides the tridentate coordinated Pd complex 7 is equally
efficient as 5 and achieves similar conversions after 24 h (enteries 5 and 6), while the ester-
functionalized NHC complex 4 displays lower activity. The different impact of bi- vs.
tridentate coordination towards conversion of ArBr vs. ArCl may be correlated with the
different rate-limiting steps depending on the halide in the ArX substrate [25]. Accordingly
the increased activity of complex 7 may be rationalized by an acceleration of the oxidative
addition step due to the presence of an anionic donor. In contrast, the presumably rate-limiting
transmetalation step in ArBr conversion is disfavored with 7, probably because of the lower
availability of coordination sites available for transmetalation. In agreement with such a
model, the flexibility of the ligand in complex 5 accommodates both, stabilization of low
oxidation states for facilitating ArCl bond activation and hemilability to provide a free
coordination site for transmetalation in the conversion of ArBr substrates.
Table 2 Catalytic Suzuki-Miyaura cross couplinga)
R
R
cat
(1 mol%)
B(OH)2
+
X
R'
R'
Entry
cat
4
X
R/R’
yield
33%
68%
30%
40%
1
2
3
4
Br
Br
Br
Cl
OMe/H
OMe/H
OMe/H
H/CHO
5
7
4
9

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5
6
Cl
Cl
H/CHO
H/CHO
64%
57%
5
7
a)
General reaction conditions: phenylboronic acid (1.2 mmol), Cs2CO3 (1.5 mmol), arylhalide (1.0 mmol) and
complex 4, 5, 7 (1 mol%) in 2-propanol (3.0 mL) at 50°C for 2 h; NMR spectroscopic yields.
Complexes 4, 5 and 7 were also tested in the Mizoroki-Heck olefination of aryl
bromides (Table 3) [26]. Higher efficiencies were obtained for substrates with a methoxy
substituent in para rather than meta position. Highest conversions were obtained with the
tridentate coordinated Pd complex 7 as catalyst precursor (entries 3 and 6). Complex 4 and 5
possessing only neutral donors gave similar conversions, which were substantially lower than
those achieved with the tridentate chelate 7 (entries 1, 2, 4, and 5). Even though the general
reaction conditions are mildly basic (NaOAc) and thus similar to those applied for the
transformation of bidentate ligand in complex 5 into a tridentate system in complex 7, the
different productivity of these two complexes implies that the catalytically active species are
different. This difference is worth noting since unlike Suzuki-Miyaura cross-coupling
reactions, Mizoroki-Heck olefination does not feature an acidic substrate for capturing the
added base. Potentially, the carboxylic acid wingtip group in 5 may change the proton transfer
(pH), thus accounting for the low conversion. Alternatively, the presence of HOAc as the
product from chelation of the carboxylate when starting with complex 5 may induce non-
productive side reactions such as hydro-dehalogenation. Attempts to improve catalysis by
abstracting the chloride ligand in complex 7 with AgBF4 only gave catalytically inactive
species and no product formation was observed even after 24 h. This result may be due to the
formation of a bimetallic system [27].
Table 3 Catalytic Mizoroki-Heck cross couplinga)
R
cat
(1 mol%)
R
+
Br
R'
R'
Entry
catalyst
X
R/R’
yield
5%
a
b
c
d
e
Br
Br
Br
Br
Br
OMe/H
OMe/H
OMe/H
H/OMe
H/OMe
4
5
7
4
5
5%
20%
30%
28%
10

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f
Br
H/OMe
60%
7
a) General reaction conditions: styrene (560 ꢀmol), NaOAc (440 ꢀmol), bromoanisole (400 ꢀmol) and palladium
complex (1 mol%) in DMA (3.0 mL) at 50°C, 2 h; NMR spectroscopic yield.
3. Conclusion
Novel chelating benzimidazolylidene palladium(II) complexes were prepared and fully
characterized. A rigidly N,C-bidentate coordinated Pd complex was obtained upon metalation
of an ester-functionalized ligand. Facile deprotection yielded a bidentate chelate with a non-
coordinating acid wingtip group. In contrast, complexation of the deprotected ligand formed a
pincer-type N,C,O-tridentate coordinated Pd complex comprising a neutral imine and an
anionic carboxylate donor. Coordination and decoordination of this carboxylate group is
triggered by the addition of base and acid, respectively. The complexes show distinctly
different activities in cross coupling reactions. The low activity of the rigidly bidentate system
suggests that the presence of a potentially coordinating carboxylate is beneficial for cross
coupling. Depending on the substrate and the type of coupling, catalytic performance
increases when the carboxylate is dangling or coordinated. The presence of different types of
donor sites (neutral and ionic) therefore constitutes an attractive concept for tailoring catalysts
in non-isohypsic processes such as cross-coupling reactions and oxidation reactions.
4. Experimental
4.1 General comments
N-methyl-1-chloromethylbenzimidazole was prepared via reported procedures [17]. All other
starting materials and reagents were obtained from commercial sources and were used as
received unless otherwise stated. NMR spectra were recorded on Varian spectrometers
operating at 400 or 500 MHz. Chemical shifts are reported in δ (ppm) relative to internal
13
Me4Si in CDCl3 or residual protio solvents. C NMR signals were assigned with the aid of
two-dimensional cross-coupling experiments. For NMR assignments, bimi refers to the 1,3-
disubstituted benzimidazole and Me-bimi refers to 1-methyl-2-substituted benzimidazole. IR
spectra were obtained on a FTIR spectrometer, and are reported here in units of cm-1.
Elemental analysis was performed on an Exeter Analytical CE440 elemental analyzer. High-
resolution mass spectrometry was carried out with a Micromass/Waters Corp. USA liquid
chromatography time-of-flight spectrometer equipped with an electrospray source.
4.2 Synthetic procedures
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Synthesis of 2. A mixture of 1 (1.8 g, 10 mmol), benzimidazole (1.18 mg, 10 mmol) and
finely ground KOH (672 mg, 12 mmol) was stirred at reflux in CH3CN (50 mL) for 48 h.
After solvent evaporation the crude solid was dissolved in CH2Cl2 (200 mL) and washed with
1 M KOH (aq.) (3 x 50 mL), brine (50 mL) and dried over MgSO . After solvent evaporation
4
the residue was purified by column chromatography (SiO2, CH2Cl2 : MeOH 8:2), or using the
KP-NH Biotage columns with 100% ethyl acetate to provide 2 (1.62 g, 62%) as white
microcrystalline powder.
1H NMR (CDCl3, 400 MHz): δ = 7.99 (s, 1H, NCHN), 7.83–7.80 (m, 2H, CHMe-bimi, CHbimi),
3
7.49 (d, 1H, JHH = 8.8 Hz, CHbimi), 7.33–7.26 (m, 5H, 3 CHMe-bimi, 2 CHbimi), 5.65 (s, 2H,
13
CH2), 3.58 (s, 3H, CH3). C{H} NMR (CDCl3, 100 MHZ): δ = 147.4 (NCN), 144.0 (Cbimi),
142.3 (Cbimi), 142.2 (CMe-bimi), 136.3 (CMe-bimi), 133.8 (Cbimi), 123.8 (CMe-bimi, CHMe-bimi
,
CHbimi), 122.9 (CHMe-bimi), 122.8 (Cbimi, CHbimi), 120.8 (CHbimi), 120.4 (CHMe-bimi), 109.6
(CHMe-bimi), 110.2 (CHbimi), 42.8 (CH2), 30.2 (CH3). Elemental analysis calcd. for C15H11N4
(262.3): C 73.26, H 5.38, N 21.36; found: C 73.23, H 5.41, N 21.38. HR-MS (m/z): 263.1284,
calcd for [M+H]+ 263.1297.
Synthesis of 3. A mixture of 2 (1.63 g, 6.2 mmol) and t-butyl bromoacetate (0.9 mL, 6.2
mmol) was refluxed in CH3CN (15 mL) for 24 h. The solvent was reduced to about 5 mL and
the product was precipitated from Et2O (50 mL). Repeated precipitation from CH2Cl2/Et2O
afforded product 3 as a white solid (1.5 g, 53%).
1H NMR (DMSO–D6, 400 MHz): δ = 9.90 (s, 1H, NCHN), 8.11–8.08 (m, 2H, Hbimi), 7.76–
3
7.69 (m, 2H, Hbimi), 7.67 (d, 1H, 3JHH = 8.0 Hz, HMe-bimi), 7.65 (d, 1H, JHH = 8.0 Hz, HMe-bimi),
3
3
7.31 (t, 1H, JHH = 7.3 Hz, HMe-bimi), 7.20 (t, 1H, JHH = 7.3 Hz, HMe-bimi), 6.34 (s, 2H, CH2),
13
5.62 (s, 2H, CH2), 3.97 (s, 3H, CH3), 1.49 (s, 9H, 3 × CH3). C{H} NMR (DMSO–D6, 100
MHz): δ = 165.5 (C=O), 147.8 (NCN), 144.2 (NCHN), 141.5 (CMe-bimi), 136.2 (CMe-bimi),
131.4 (Cbimi), 131.2 (Cbimi), 127.0 (CHbimi), 126.9 (CHbimi), 122.8 (CHbimi), 122.0 (CHMe-bimi),
118.9 (CHbimi), 114.2 (CHMe-bimi), 113.9 (CHMe-bimi), 110.4 (CHMe-bimi), 83.3 (CMe3), 48.2
(CH2), 43.5 (CH2), 30.1 (CH3), 27.7 (CH3). Elemental analysis calcd. for C22H25N4O2Br
(456.11) × 0.5 CH2Cl2: C 54.07, H 5.24, N 11.21; found: C 53.83, H 5.34, N 11.09. HR-MS
(m/z): 377.1995, calcd for [M–Br]+ 377.1977. IR: (CO) 1740 cm–1.
Synthesis of 4. A mixture of 3 (1.5 g, 3.63 mmol), NaOAc (0.04 g, 4.87 mmol) and PdCl2
(0.60 g, 0.32 mmol) was refluxed in CH3CN (20 mL) for 24 h. The solution was filtered
through a pad of Celite and the solvent reduced to 5 mL. Et2O (25 mL) was added which
12

ACCEPTED MANUSCRIPT
precipitated the product as a yellow powder. The residue was purified by flash column
chromatography (SiO2, CH2Cl2/CH3CN 1:9). Crystals were grown by diffusion of Et2O into a
solution of 4 dissolved in CHCl3 (1.0 g, 50%).
3
1H NMR (DMSO–D6, 500 MHz): δ =8.31 (d, 1H, JHH = 8.1 Hz, HMe-bimi), 8.14 (d, 1H, 3JHH
=
3
3
8.1 Hz, HMe-bimi), 7.68 (d, 1H, JHH = 8.1 Hz, Hbimi), 7.65 (d, 1H, JHH = 8.1 Hz, Hbimi), 7.47 (t,
1H, 3JHH = 7.5 Hz, HMe-bimi), 7.41 (t, 1H, 3JHH = 7.5 Hz, Hbimi), 7.37 (t, 1H, 3JHH = 7.5 Hz, HMe-
3
2
2
bimi), 7.32 (t, 1H, JHH = 8.0 Hz, Hbimi), 6.37 (d, 1H, JHH = 16.7 Hz, CH2), 6.08 (d, 1H, JHH
=
2
2
16.7 Hz, CH2), 5.79 (d, 1H, JHH = 17.3 Hz, CH2), 5.58 (d, 1H, JHH = 17.3 Hz, CH2), 4.09 (s,
13
3H, CH3), 1.37 (s, 9H, 3 × CH3). C{H} NMR (DMSO–D6, 125.5 MHz): δ = 176.0 (C=O),
166.6 (C-NHC), 148.4 (CMe-bimi), 138.8 (Cbimi), 134.1 (Cbimi), 133.8 (CMe-bimi), 132.8 (NCN),
124.2 (CHbimi), 124.1 (CHbimi),123.1 (CHMe-bimi), 119.9 (CHbimi), 112.0 (CHMe-bimi), 111.8
(CHMe-bimi), 111.7 (CHMe-bimi), 109.6 (CHbimi), 49.0 (CH2), 42.7 (CH2), 31.5 (CH3), 27.2 (3 ×
CH3). Elemental analysis calcd. for C22H24Br2N4O2Pd (692.68): C 41.11, H 3.76, N 8.72;
found: C 41.93, H 3.63, N 8.83. IR: (CO) 1740 cm–1.
Synthesis of 5. Complex 4 (0.1 g, 0.2 mmol) was dissolved in CH2Cl2 (10 mL). TFA (0.1 mL,
2 mmol) was added dropwise in an ice bath. The solution was allowed to warm to room
temperature and stirred for 4 h. Et2O (30 mL) was added to precipitate the product 5 as a
brown solid. The residue was purified by flash column chromatography (SiO2,
CH2Cl2/CH3CN 1:9) to yield 5 as an off-white solid (0.07 g, 78%)
1H NMR (DMSO–D6, 500 MHz): δ = 8.11 (d, 1H, 3JHH = 8.3 Hz, HMe-bimi), 8.03 (d, 1H, 3JHH
=
3
3
7.8 Hz, HMe-bimi), 7.63 (d, 1H, JHH = 7.3 Hz, Hbimi), 7.58 (d, 1H, JHH = 8.6 Hz, Hbimi), 7.45 (t,
1H, 3JHH = 8.3 Hz, HMe-bimi), 7.45 (m, 2H, Hbimi), 7.33 (t, 1H, 3JHH = 7.8 Hz, HMe-bimi), 6.19 (d,
1H, 2JHH = 16.6 Hz, CH2), 5.96 (d, 1H, 2JHH = 16.6 Hz, CH2), 5.83 (d, 1H, 2JHH = 18.5 Hz, CH2),
5.50 (d, 1H, 2JHH = 18.5 Hz, CH2), 4.02 (s, 3H, CH3). 13C{H} NMR (DMSO–D6, 125.5 MHz):
δ = 174.0 (C=O), 168.6 (C-NHC), 148.8 (CMe-bimi), 137.4 (Cbimi), 134.5 (Cbimi), 133.7 (CMe-
bimi), 132.4 (NCN), 123.4 (CHbimi), 123.3 (CHbimi), 120.6 (CHMe-bimi), 120.1(CHbimi), 113.8
(CHMe-bimi), 112.9 (CHMe-bimi), 112.5 (CHMe-bimi), 112.0(CHbimi), 50.7 (CH2), 43.4 (CH2), 32.0
(CH3). Elemental analysis calcd. for C20H16ClF3N4O4Pd (573.98) × 0.5 CH2Cl2: C 39.86, H
2.77, N 9.07; found: C 39.84, H 3.07, N 8.90. HR-MS (m/z): 460.9839, calcd for [M–TFA]+
460.9996. IR: 1678 cm–1.
Synthesis of 6. Compound 3 (0.5 g, 1 mmol) was dissolved in CH2Cl2 (10 mL) in an ice bath.
Trifluoroacetic acid (3.5 mL, 43 mmol) was added dropwise to the solution, which was left
13

ACCEPTED MANUSCRIPT
stirring for 16 h. The solvent was removed and the product was re-dissolved in CH2Cl2 (5
mL) and precipitated from Et2O (100 mL) to afford product 6 as a white solid (0.27 g, 48%).
1H NMR (DMSO–D6, 400 MHz): 9.92 (s, 1H, NCHN), 8.08–8.06 (m, 2H, CHbimi), 7.79–7.67
(m, 2H, CHbimi), 7.65 (d, 1H, 3JHH = 8.2 Hz, CHMe-bimi), 7.53 (d, 1H, 3JHH = 8.2 Hz, CHMe-bimi),
3
3
7.31 (t, 1H, JHH = 7.2 Hz, CHMe-bimi), 7.22 (t, 1H, JHH = 7.2 Hz, CHMe-bimi), 6.31 (s, 2H,
CH2), 5.57 (s, 2H, CH2), 3.95 (s, 3H, CH3). 13C{H} NMR (DMSO–D6, 100 MHz): δ = 168.2
(C=O), 147.9 (NCN), 144.5 (NCHN), 141.0 (CMe-bimi), 136.6 (CMe-bimi), 131.9 (Cbimi), 131.5
(Cbimi), 127.4 (CHbimi), 127.3 (CHbimi), 123.5 (CHMe-bimi), 122.8 (CHbimi), 119.0 (CHMe-bimi),
114.4 (CHbimi), 114.3 (CHbimi), 110.0 (CHMe-bimi), 48.1 (CH2), 43.6 (CH2), 30.6 (CH3).
Elemental analysis calcd. for C18H17N4O2Br (322.14) × 0.25 CH2Cl2 × 0.25 Et2O: C 52.53, H
4.07, N 12.71; found: C 52.43, H 4.57, N 12.70. HR-MS (m/z): 321.1346, calcd for [M–Br]+
321.1351. IR: (CO) 1741 cm–1.
Synthesis of 7. Complex 7 was prepared using the same procedure as for 4 but starting from
ligand 6 (1.0 g, 2.4 mmol), NaOAc (0.26 g, 3.2 mmol) and PdCl2 (0.42 g, 2.4 mmol).
Complex 7 was obtained as a yellow powder (0.70 g, 63%).
1H NMR (DMSO–D6, 500 MHz): δ = 8.43 (d, 1H, 3JHH = 8.2 Hz, CHMe-bimi), 8.19 (d, 1H, 3JHH
= 8.0 Hz, CHbimi), 7.82 (d, 1H, 3JHH = 8.0 Hz, CHbimi), 7.65 (d, 1H, 3JHH = 8.2 Hz, CHMe-bimi),
7.42 (t, 1H, 3JHH = 7.4 Hz, CHbimi), 7.39 (t, 1H, 3JHH = 7.8 Hz, CHbimi), 7.33 (t, 1H, 3JHH = 7.4
3
Hz, CHMe-bimi), 7.23 (t, 1H, JHH = 7.8 Hz, CHMe-bimi), 6.08 (s, 2H, CH2), 5.08 (s, 2H, CH2),
13
4.00 (s, 3H, CH3). C{H} NMR (DMSO–D6, 125.5 MHZ): δ = 168.6 (C=O), 156.3 (Ccarb),
148.0, 134.4, 134.2 (3 × CMe-bimi), 132.6, 124.6 (2 × Cbimi), 124.4, 124.3, 124.1, 122.7 (4 ×
CHMe-bimi), 112.5, 112.4, 112.2, 120.5 (4 × CHbimi), 50.9 (CH2), 43.4 (CH2), 32.1 (CH3).
Elemental analysis calcd. for C18H15ClN4O2Pd (459.99) × CH2Cl2: C 41.78, H 3.14, N 10.26;
found: C 41.82, H 3.27, N 9.48. Despite repetitive precipitations, the %N remained too high.
HR-MS (m/z): 458.9748, calculated for [M–H]+ 458.9840. IR: 1624 cm–1.
4.3 Representative catalytic procedures
Suzuki cross coupling catalysis: Catalyst (1 mol%), phenylboronic acid (147 mg, 1.2 mmol),
Cs2CO3 (0.48 g, 1.5 mmol), 4-chlorobenzaldehyde or 3-bromoanisole (1.0 mmol), 2-propanol
(3 mL) were added to a schlenk flask equipped with magnetic stirrer. The mixture was placed
in a pre-heated oil bath at 50 °C for 24 h. The mixture was quenched with H2O (25 mL) and
extracted with CH2Cl2 (3 × 25 mL). The combined organic layers were dried over MgSO4,
filtered, and evaporated in vacuo.
14

ACCEPTED MANUSCRIPT
Heck cross coupling catalysis: Catalyst (1 mol%), styrene (64 ꢀL, 560 ꢀmol), NaOAc (36 mg,
440 ꢀmol), bromoanisole (400 ꢀmol), and DMA (3 mL) were added to a schlenk flask
equipped with magnetic stirrer. The reaction mixture was placed in a pre-heated oil bath at
110 °C for 24 h. The mixture was quenched with H2O (25 mL) and extracted with CH2Cl2 (3
× 25 mL). The combined organic layers were dried over MgSO4, filtered, and evaporated in
vacuo.
4.4 Crystallographic details
Crystal data for 4 were collected using an Agilent Technologies SuperNova A diffractometer
fitted with an Atlas detector and using monochromated Mo–Kα radiation (0.71073 Å). A
complete dataset was collected, assuming that the Friedel pairs are not equivalent. An
analytical numeric absorption correction was performed [24]. The structure was solved by
direct methods using SHELXS-97 [29] and refined by full matrix least-squares on F2 for all
data using SHELXL-97 [29]. Hydrogen atoms were added at calculated positions and refined
using a riding model. Their isotropic thermal displacement parameters were fixed to 1.2 times
(1.5 times for methyl groups) the equivalent one of the parent atom. Anisotropic thermal
displacement parameters were used for all non-hydrogen atoms. Disordered solvent was
treated with the SQUEEZE procedure as implemented in PLATON [30]. Further
crystallographic details are compiled in Table 4. CCDC number 999440 contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Table 4 Crystallographic data for complex 4
Crystal size / mm
Empirical formula
Fw
0.12 × 0.08 × 0.05
C23.50 H27.25 N4.25 O2.25 Cl1.25 Br0.75 Pd
615.84
T /K
100(2)
Crystal system
Space group
Unit cell
Triclinic
P–1 (#2)
a /Å
b /Å
c /Å
α /°
β /°
γ /°
10.08185(8)
10.54498(11)
13.42923(16)
74.1843(9)
71.6551(9)
89.8778(7)
1298.36(2)
2
Volume /Å3
Z
Dcalcd /g cm–3
ꢀ /mm–1
F(000)
1.575
2.024
619
15

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Reflections collected / R(int)
Max. and min. transmission
Data / restraints / parameters
Goodness–of–fit on F2
23543 / 0.0243
0.968 – 0.928
6444 / 0 / 286
1.073
R1 = 0.0241, wR2 = 0.0624
R1 = 0.0285, wR2 = 0.0642
0.479, –0.470
Final R indices for I>2
R indices for all data
σ(I)
Largest diff. peak, hole /e Å–3
5. Acknowledgement
We thank the European Research Council (StG 208651 and CoG 615653) and Science
Foundation Ireland (10/RFP/CHS2844) for financial support and Johnson Matthey for a
generous metal loan.
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Highlights
•
•
•
Pincer type N,C,O-tridentate carbene palladium complexes were synthesized
Hemilabile O-donor site binding and dissociation is demonstrated
Catalytic activity in cross-coupling depends on the ligand coordination mode
Graphical Synopsis
O
N
N
OH
N
N
O
base
acid
N
N
N
X
Pd
O
N
Pd
X
X
active Suzuki catalytst with ArBr and ArCl
inactive Heck catalyst
active Suzuki catalytst with ArCl only
active Heck catalyst
Pincer-type N-heterocyclic carbene palladium complexes featuring a N,C,O-tridentate ligand
with a hemilabile oxygen donor display diverging catalytic activity that is in parts controlled
by the (de)coordination of the oxygen donor.
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