Molecular structures and catalytic activity of palladium complexes derived from lutidine-bridged bis(benzimidazolin-2-ylidene) ligands

Article abstract of DOI:10.1515/znb-2007-0308

Reaction of lutidine-bridged dibenzimidazolium dibromides 1-4 with palladium acetate gives pincer-type palladium complexes of the type [Pd(L)Br]Br [5]Br-[8]Br. Crystals suitable for an X-ray diffraction study have been obtained by slow evaporation of the solvent from dichloromethane/methanol solutions of [7]Br and [8]Br. The crystal structure of [7]⁺ reveals a pincer topology of the cationic complex with a distorted square-planar coordination geometry at the metal center. From a solution of complex [8]Br, a dinuclear byproduct [9]⁺ was obtained with two bis(benzimidazolin2-ylidene) ligands coordinating in a bridging fashion. The pincer-type palladium complexes [5]Br[8]Br were tested as precatalysts in Suzuki coupling reactions.

Full text of DOI:10.1515/znb-2007-0308

Molecular Structures and Catalytic Activity of Palladium Complexes
Derived from Lutidine-bridged Bis(benzimidazolin-2-ylidene) Ligands
Mareike C. Jahnke, Tania Pape, and F. Ekkehardt Hahn
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-Universita¨t Mu¨nster,
Corrensstraße 36, D-48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. F. E. Hahn. Fax +49 251 833 3108. E-mail: fehahn@uni-muenster.de
Z. Naturforsch. 2007, 62b, 357 – 361; received November 29, 2006
Dedicated to Prof. Helgard G. Raubenheimer on the occasion of his 65th birthday
Reaction of lutidine-bridged dibenzimidazolium dibromides 1 – 4 with palladium acetate gives
pincer-type palladium complexes of the type [Pd(L)Br]Br [5]Br–[8]Br. Crystals suitable for an X-ray
diffraction study have been obtained by slow evaporation of the solvent from dichloromethane/meth-
anol solutions of [7]Br and [8]Br. The crystal structure of [7]+ reveals a pincer topology of the
cationic complex with a distorted square-planar coordination geometry at the metal center. From a
solution of complex [8]Br, a dinuclear byproduct [9]+ was obtained with two bis(benzimidazolin-
2-ylidene) ligands coordinating in a bridging fashion. The pincer-type palladium complexes [5]Br–
[8]Br were tested as precatalysts in Suzuki coupling reactions.
Key words: Heterocyclic Carbene Complexes, Palladium(II) Complexes, Pincer Complexes
Introduction
type of N-heterocyclic carbene between the saturated
imidazolidin-2-ylidene and the ubiquitous unsaturated
imidazolin-2-ylidene [1, 2]. We have described some
palladium pincer complexes derived from lutidine-
[8a] or phenylene-bridged [8b] bis(benzimidazolin-2-
ylidene) ligands. These complexes have been tested as
precatalysts in Heck-type coupling reactions.
Here we present an additional molecular structure
of a pincer complex with a lutidine-bridged bis(benz-
imidazolin-2-ylidene) ligand. In addition, the unusual
molecular structure of a dinuclear complex with a
pincer-type ligand bridging two metal centers is de-
scribed. The catalytic activity of the pincer complexes
in Suzuki-type coupling reactions is also discussed.
Stable N-heterocyclic carbenes and particularly
their metal complexes have recently attracted atten-
tion owing to their application in various homoge-
neous catalytic processes [1]. Several procedures for
the preparation of complexes with benzimidazolin-2-
ylidene ligands have been reported. Such complexes
can be obtained by reaction of a transition metal com-
plex with the free carbene [2] or by cleavage of the
olefinic C=C double bond of dibenzotetraazafulvalenes
with a coordinativelyunsaturated transition metal com-
plex [3]. In addition, complexes with benzimidazo-
lin-2-ylidene ligands can be prepared by reaction of
benzimidazolium salts with transition metal complexes
like Pd(OAc)2 [4], [Ir(µ-OMe)(cod)]2 [5] or Ag2O [6]
via an in situ deprotonation of the benzimidazolium
salts followed by complex formation. Finally, com-
plexes with benzannulated N-heterocyclic carbene lig-
ands are obtained by intramolecular cyclization of
coordinated β-functionalized isocyanides giving the
NH,NH-stabilized carbene ligands which are easily
N,Nꢀ-dialkylated [7].
Results and Discussion
We recently prepared the lutidine-bridged bis(benz-
imidazolin-2-ylidene) ligands 1 – 4 and reported the
synthesis of their pincer-type palladium complexes
[5]Br–[8]Br [8a]. These complexes were obtained by
in situ deprotonation of the dibenzimidazolium salts
with palladium acetate in good yield (Scheme 1).
We became interested in the coordination chemistry This method is similar to the published procedure for
and application in catalysis of pincer-type complexes the preparation of lutidine-bridged dicarbene palla-
with a benzimidazolin-2-ylidene donor function as a dium complexes with imidazolin-2-ylidene donors [9].
carbene source due to the intermediate position of this Complexes [5]Br–[8]Br have been completely charac-
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358
M. C. Jahnke et al. · Molecular Structures and Catalytic Activity of Palladium Complexes
Scheme 1. Synthesis of pin-
cer-type palladium complexes
[5]Br–[8]Br.
Scheme 2. Synthesis of compound [8]Br and the formation
Fig. 1. Molecular structure of the cation [7]+ in
[7]Br·2MeOH. Hydrogen atoms have been omitted for clar-
ity. Displacement ellipsoids are drawn at the 50 % prob-
of byproduct [9]Br.
distances in [7]+ are slightly elongated compared to
the values found for neutral palladium complexes with
two benzimidazolin-2-ylidene donors in a cis config-
˚
ability level. Selected bond lengths (A) and angles (deg):
Pd–C1 2.025(2), Pd–C15 2.027(3), Pd–N3 2.048(2), Pd–Br
2.4111(4), C1–N1 1.352(3), C1–N2 1.350(3), C15–N4
1.353(3), C15–N5 1.345(3); Br–Pd–N3 179.16(6), Br–Pd–
C1 93.06(7), Br–Pd–C15 94.58(7), N3–Pd–C1 87.12(9),
N3–Pd–C15 85.21(9), C1–Pd–C15 171.98(10), N1–C1–
N2 106.7(2), N4–C15–N5 106.6(2), N1–C8–C9 111.4(2),
N4–C14–C13 109.3(2).
˚
uration [1.987(4)– 1.991(7) A] [4a]. As reported for
several other dicarbene pincer complexes, the C–Pd–C
bond angle reveals the greatest deviation from a perfect
square-planar arrangement of the four donor functions
at the metal center [9] owing to the sterical constraint
imposed by the lutidine spacer group. The bond an-
gles at the bridging methylene carbon atoms remain
close to tetrahedral [N1–C8–C9 111.4(2)◦, N4–C14–
C13 109.3(2)◦]. As seen with other benzimidazolin-2-
ylidene complexes, the N–C–N angle within the car-
bene donor expands from about 104◦ for the free lig-
and [2a] to about 107◦ for the coordinated ligand.
Two compounds were obtained upon attempts to
crystallize the salt [8]Br (Scheme 2). Apart from the
expected mononuclear pincer complex [8]+ in [8]Br
a dinuclear species [9]+ was obtained as a byprod-
uct (Scheme 2), which had not been detected by NMR
spectroscopy prior to the crystallization experiments.
The dinuclear complex [9]+ contains the potentially
terized by NMR spectroscopy and mass spectrometry.
Isolation of the free carbene ligands or their dibenzote-
traazafulvalene dimers was not possible because of in-
tramolecular rearrangement processes occurring upon
C2 deprotonation as described earlier [10].
Crystals of [7]Br·2MeOH suitable for an X-ray
diffraction analysis were obtained by recrystalliza-
tion of [7]Br from a dichloromethane/methanol solu-
tion. The cationic palladium complex [7]+ exhibits a
slightly distorted square-planar coordination geome-
˚
try (Fig. 1). The Pd–C bond lengths of 2.025(2) A
˚
(Pd–C1) and 2.027(3) A (Pd–C15) fall in the range
previously reported for similar bis(benzimidazolin-2-
ylidene) pincer complexes [8] and neutral palladium
complexes with two benzimidazolin-2-ylidene ligands tridentate dicarbene ligand acting in a bridging fashion.
in trans configuration [4b]. In addition, the Pd–C bond One palladium atom (Pd1) is surrounded by two benz-
lengths in [7]+ are similar to equivalent bond param- imidazolin-2-ylidene donors in trans orientation in ad-
eters found in palladium pincer-type complexes with dition to two bromo ligands. The second palladium
imidazolin-2-ylidene donor functions [9]. The Pd–C atom (Pd2) is coordinated by two benzimidazolin-2-
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M. C. Jahnke et al. · Molecular Structures and Catalytic Activity of Palladium Complexes
359
Table 1. Pd-Catalyzed Suzuki coupling re-
action of phenylboronic acid with aryl
bromides.a
a Reaction conditions: 1.0 mmol of the aryl bromide,
1.2 mmol of phenylboronic acid, 2 mmol K2CO3,
Pd-catalyst and 50 mL of toluene. Yields were de-
termined by gas chromatography.
entry
catalyst
mol% cat.
0.1
R
H
H
H
H
H
H
time [h]
yield [%]
1
2
3
4
5
6
7
5
6
7
8
5
8
8
24
24
24
24
24
2
76
69
70
69
37
48
61
0.1
0.1
0.1
0.01
0.01
0.01
C(O)CH3
2
ylidene donors from different ligands in a cis arrange-
ment together with one lutidine nitrogen atom and one
bromo ligand. This arrangement leads to two different
sets of Pd–C bond lengths in [9]+.
The Pd–C bond lengths involving palladium atom
Pd1 (Fig. 2) with the trans orientation of the car-
bene donor functions are comparable to those ob-
served in the pincer-type palladium complex [7]+. The
Pd–C bonds at Pd2 with a cis arrangement of the two
benzimidazolin-2-ylidene donor functions are shorter
˚
˚
[Pd2–C15 1.964(7) A, Pd–C31 1.988(6) A] and com-
pare well to the Pd–C bond lengths found in dicar-
bene complexes with a cis arrangement of the car-
bene donor functions [3b, 4a]. Differences between the
metal centers were also observed by comparison of
the Pd–Br bond lengths. At Pd1 the two bromo lig-
ands in the trans arrangement exhibit Pd1–Br1 and
Fig. 2. Molecular structure of the cation [9]+ in
[9]Br·H2O·MeOH. Hydrogen atoms, the bromide anion and
the solvent molecules have been omitted for clarity. Dis-
placement ellipsoids are drawn at the 50 % probability
˚
level. Selected bond lengths (A) and angles (deg): Pd1–Br1
2.4476(8), Pd1–Br2 2.4359(8), Pd1–C1 2.022(6), Pd1–C45
2.023(6), Pd2–Br3 2.4828(9), Pd2–N8 2.106(5), Pd2–C15
˚
Pd1–Br2 bond lengths of 2.4476(8) and 2.4359(8) A. 1.964(7), Pd2–C31 1.988(6), C1–N1 1.361(8), C1–N2
˚
1.342(7), C15–N4 1.355(8), C15–N5 1.359(9), C31–N6
1.348(7), C31–N7 1.347(8), C45–N9 1.350(7), C45–N10
1.341(8); Br1–Pd1–Br2 177.90(3), Br1–Pd1–C1 88.6(2),
Br1–Pd1–C45 89.8(2), Br2–Pd1–C1 92.1(2), Br2–Pd1–C45
89.5(2), C1–Pd1–C45 178.2(2), Br3–Pd2–N8 93.53(13),
Br3–Pd2–C15 87.0(2), Br3–Pd2–C31 165.6(2), N8–Pd2–
C15 179.4(3), N8–Pd2–C31 86.4(2), C15–Pd2–C32 93.0(3),
N1–C1–N2 106.5(5), N4–C15–N5 107.6(6), N6–C31–N7
106.9(5), N9–C45–N10 105.5(5).
The Pd2–Br3 bond [2.4828(9) A] is longer which re-
flects the stronger trans effect of the carbene donor
function compared to the bromo ligand at Pd1. The co-
ordination geometry at the two palladium atoms is best
described as distorted square-planar.
We have previously described the catalytic activ-
ity of palladium pincer-complexes which were gen-
erated in situ by reaction of one of the dibenzimida-
zolium salts 1 – 4 with PdCl2 in DMF [8a]. Here we re-
port on the catalytic activity of the isolated palladium
complexes [5]Br–[8]Br as precatalysts in the Suzuki
coupling reaction of bromobenzene and phenylboronic
acid. A catalyst loading of 0.1 mol-% gave 69 – 76 %
conversion after 24 h in refluxing toluene (Table 1).
reaction of the more reactive 4-bromoacetophenone
and phenylboronic acid proceeds with 61 % conver-
sion within 2 h with precatalyst [8]Br. This value is
comparable to that of other palladium carbene com-
plexes with benzimidazolin-2-ylidenedonor functions.
Furthermore, the observed conversion rate is higher
than found for neutral pincer-type palladium com-
plexes with xylene-bridged bis(benzimidazolin-2-yl-
idene) ligands which we reported recently [8b].
The use of a smaller amount of precatalyst [5]Br
(0.01 mol-%) yielded only 37 % conversion after 24 h.
In contrast to [5]Br, the palladium complex [8]Br
exhibits higher catalytic activity, with 49 % conver-
We have described an unusual coordination mode of
sion after a reaction time of only 2 h. The coupling a lutidine-bridged bis(benzimidazolin-2-ylidene) lig-
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360
M. C. Jahnke et al. · Molecular Structures and Catalytic Activity of Palladium Complexes
and leading to a dinuclear palladium complex with finement [12] were achieved with standard Patterson and
Fourier techniques. All non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen atoms
were added to the structure models in calculated positions.
Selected crystallographic details for [7]Br·2MeOH: For-
mula C29H37N5Br2O2Pd, M = 753.86, colourless crystal,
0.21 × 0.18 × 0.11 mm3, monoclinic, space group P21/c,
two bridging ligands. Both metal centers in this com-
plex are coordinated by two benzimidazolin-2-ylidene
donors, but with a different arrangement (cis or trans)
of the carbene donors. Attempts to use the palladium
complexes [5]Br–[8]Br as precatalysts in Suzuki-type
reactions showed in most cases an acceptable catalytic
activity in a range typically observed for neutral dicar-
bene pincer complexes.
˚
a = 17.3◦15(3), b = 9.2733(15), c = 19.470(3) A, β =
3
99.202(3) , V = 3086.0(8) A , ρcalcd = 1.62 g cm−3, µ =
˚
3.2 mm−1, empirical absorption correction (0.5508 ≤ T ≤
0.7180), Z = 4, 34191 intensities collected ( h, k, l),
8950 independent (Rint = 0.0458) and 6878 observed inten-
sities [I ≥ 2σ(I)], 358 refined parameters, residuals for all
data R = 0.0574, wR2 = 0.0880, max. residual electron den-
Experimental Section
The dibenzimidazolium dibromides 1 – 4 and the dicar-
bene palladium complexes [5]Br – [8]Br were prepared as
previously described [8a].
−3
˚
sity 0.91/−0.47 e A
.
Selected crystallographic details for [9]Br·H2O·MeOH:
Formula C59H72N10Br4O2Pd2, M = 1485.71, colourless
crystal, 0.24 × 0.11 × 0.07 mm3, triclinic, space group
General method for the Suzuki coupling: A two-necked
100 mL flask was fitted with a reflux condenser and
charged with an aryl bromide (1.0 mmol), phenylboronic
acid (0.144 g, 1.2 mmol) and potassium carbonate (0.276 g,
2.0 mmol). Toluene (50 mL) was added and the suspen-
sion was brought to reflux. The desired amount of the pal-
ladium complexes [5]Br–[8]Br was dissolved in 1 mL of
dichloromethane. This solution was injected into the boiling
toluene solution, and the mixture was refluxed for the de-
sired time. The mixture was allowed to cool to r. t. and the
organic phase was passed over a short silica gel column. The
resulting solution was then analyzed by quantitative GC anal-
ysis with a Shimadzu GC-2100 with an Aglient Technologies
HP 5 capillary column (30 m).
¯
˚
P1, a = 13.775(2), b = 14.842(2), c = 17.275(2) A,
α = 106.779(3), β = 93.275(3), γ = 104.697(3)◦, V =
3238.2(8) A , ρcalcd = 1.52 g cm−3, µ = 3.0 mm−1, em-
3
˚
pirical absorption correction (0.5262 ≤ T ≤ 0.8138), Z =
2, 31689 intensities collected ( h, k, l), 14861 in-
dependent (Rint = 0.0432) and 9373 observed intensities
[I ≥ 2σ(I)], 723 refined parameters, residuals for all data
R = 0.1037, wR2 = 0.1819, max. residual electron den-
−3
˚
sity 1.33/−0.79 e A
.
Crystallographic data (excluding structure factors) have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication CCDC 629034 for
[7]Br·2MeOH and CCDC 629035 for [9]Br·H2O·MeOH.
Copies of the data can be obtained free of charge via www.
ccdc.cam.ac.uk/data request/cif.
X-Ray structure determination of compounds [7]Br ·
2MeOH and [9]Br · H2O · MeOH: Suitable crystals of
[7]Br·2MeOH and [9]Br·H2O·MeOH were mounted on a
Bruker AXS 2000 CCD diffractometer equipped with a ro-
˚
tating molybdenum anode (λ = 0.71073 A) and a CCD area
Acknowledgement
detector. Diffraction data were measured at 153(2) K in the
range 2.4 ≤ 2θ ≤ 60.1◦ for [7]Br·2MeOH and 3.1 ≤ 2θ ≤
55.2◦ for [9]Br·H2O·MeOH. Structure solution [11] and re-
Financial support by the Deutsche Forschungsgemein-
schaft (SFB 424 and IRTG 673) is gratefully acknowledged.
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