Synthesis and characterization of a 4-nitrophenyl functionalized NHC ligand and its palladium(II) complex

Article abstract of DOI:10.1016/j.jorganchem.2013.05.025

The synthesis and characterization of a novel pyridine coordinated dichloridopalladium(II) NHC complex is described. This compound possesses a saturated NHC ligand that is substituted with a para-nitrophenyl function in the 4-position of the NHC backbone.

Full text of DOI:10.1016/j.jorganchem.2013.05.025

Journal of Organometallic Chemistry xxx (2013) 1e7
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of a 4-nitrophenyl functionalized NHC
ligand and its palladium(II) complex
,
b
,
a
a
c
c
Fatemeh Rajabi a
, Jens Trampert , Yu Sun , Mark Busch , Stefan Bräse ,
**
Werner R. Thiel a
,
*
a Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Str. 54, 67663 Kaiserslautern, Germany
b Department of Science, Payame Noor University, P.O. Box 19395-4697, Tehran, Iran
c Karlsruher Institut für Technologie, Institut für Organische Chemie, Fritz-Haber-Weg 6, Geb. 30.42, 76131 Karlsruhe, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of a novel pyridine coordinated dichloridopalladium(II) NHC complex
is described. This compound possesses a saturated NHC ligand that is substituted with a para-nitrophenyl
function in the 4-position of the NHC backbone. It is accessible in four steps with overall good yields
starting from 4-nitro acetophenone.
Received 11 March 2013
Received in revised form
21 May 2013
Accepted 22 May 2013
Ó 2013 Elsevier B.V. All rights reserved.
Dedicated to Prof. Dr. Dr. H.C. Mult. Wolf-
gang A. Herrmann on the occasion of his
65th birthday.
Keywords:
NHC ligand
Palladium
Nitro groups
X-ray structure
1. Introduction
bulky substituents at the nitrogen atoms for stabilization, or by in-
situ deprotonation performed by a basic ligand e mostly a car-
About 20 years ago N-heterocyclic carbenes (NHCs) were
introduced as highly versatile and powerful alternatives for the
widely used but air-sensitive phosphine ligands in catalysis [1].
boxylato ligand e coordinated to the metal precursor [7]. Recently
NHC silver complexes generated by the reaction of basic silver
oxide with an imidazolium or an imidazolinium salt turned out to
be versatile NCH transfer reagents when subsequently being reac-
ted with an appropriate metal containing fragment [8]. Compared
to unsaturated NHC ligands derived from imidazolium precursors,
saturated NHCs derived from imidazolinium salts are slightly more
basic [9], leading to an increased activity and efficiency in certain
catalytic reactions [10]. The bulkiness of the substituents at the
nitrogen atoms of the NHC further influences the stabilities and the
catalytic activities of the derived transition metal complexes. In Ce
C-coupling reactions as well as in olefin metathesis, bulky sub-
stituents at the NHC ligand are a prerequisite for catalytic activity.
Therefore recently considerable efforts have in particularly been
achieved to develop bulky NHC ligands [3b,11].
They turned out to be strong
s-donors but poor p-acceptors [2]
leading to high chemical and thermal stabilities of especially no-
ble metal complexes which often show better activities in catalytic
transformations than the corresponding phosphine complexes.
This is of special importance in palladium catalyzed coupling re-
actions [3] and in ruthenium catalyzed olefin metathesis [4], where
the use of NHC coordinated complexes led to a tremendous in-
crease in performance.
The synthesis of an NHC ligand generally involves the depro-
tonation of an imidazolium or an imidazolinium precursor by ac-
tion of a base [5]. This can either be achieved by formation and
isolation of the free carbene ligand [6], which generally requires
We have been interested in the immobilization of well-defined
transition metal catalysts by binding them covalently or by elec-
trostatic interactions onto the surfaces of inorganic support mate-
rials [12]. For this purpose, a well-established ligand system is
functionalized by a linker unit that enables the grafting process. We
* Corresponding author. Tel.: þ49 631 2052752.
** Corresponding author. Fachbereich Chemie, Technische Universität Kai-
serslautern, Erwin-Schrödinger-Str. 54, 67663 Kaiserslautern, Germany.
E-mail addresses: f_rajabi@pnu.ac.ir (F. Rajabi), thiel@chemie.uni-kl.de (W.R. Thiel).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.05.025
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F. Rajabi et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
here present the facile introduction of a nitrophenyl unit into the
backbone of an imidazolinium salt and the synthesis of the corre-
sponding palladium(II) NHC complex. The nitrophenyl group will in
the future be used to immobilize such NHC ligands and their
catalytically active transition metal complexes. To the best of our
knowledge, this is the first attempt to prepare a non-symmetrically
4-aryl-substituted NHC precursor, which additionally enables the
investigation of the electronic properties of such a system.
species present in a ratio of about 3.5:1. This is due to the hindered
rotation around the C]N bonds. The resonances of the protons in
the ortho- and the meta-positions of the nitrophenyl group of the
major component are found at one single chemical shift
(d
¼ 8.34 ppm) in the 1H NMR spectrum, while the same moiety of
the minor isomer shows the two expected doublets at 8.13 and
7.42 ppm. The 1H NMR resonances of the CH]N groups of the two
isomers appear as singlets at 8.34 and 7.86 ppm, while the 13C NMR
resonances of the two imine groups of the major isomer are
observed at 163.8 (CH]N) and 163.0 ppm (C]N). For the reduction
of 2 to the corresponding diamine 3 we tried a series of different
reducing agents, but solely NaBH3(CN) allowed the isolation of the
desired product in high yields [16]. The formation of 3 is clearly
proved in the 1H NMR spectrum by the resonances of the ethylene
protons. Two diastereotopic protons of the CH2 group lead to two
doublets of doublets at 3.55 and 3.67 ppm and the proton of the CH
group gives another doublet of doublet at 4.77 ppm, while the 13C
NMR resonances of these groups are observed at 62.6 and
54.6 ppm. The ring closure reaction leading to the imidazolinium
salt was carried out with triethylorthoformiate and an appropriate
ammonium salt [17], providing the target compounds 4aec
differing in the counter anion in more than 50e80% yields [18].
Their NMR spectroscopic data are quite similar. Therefore solely the
features of compound 4a with chloride as the counter anion shall
here be discussed. The proton in the 2-position of the imidazoli-
nium core gives a characteristic peak at 9.43 ppm in the 1H NMR
spectrum. Doublets at 8.02 and 7.52 ppm (JH,H ¼ 8.7 Hz) are
observed for the protons of the nitro phenyl ring, and four quite
broad singlets between 6.59 and 6.91 ppm are assigned to the CH
units of the mesityl substituents. Three doublets of doublets at 6.48,
5.20 and 4.40 ppm are observed for the protons of the imidazoli-
nium backbone, all are strongly shifted to lower field compared to
compound 3. In the aliphatic region, there are two sharp singlets
which we assign to the methyl groups in the para-position and two
broad resonances representing the ortho-methyl substituents. From
these data it is clear, that the rotation around the NeC bond is
2. Results and discussion
Since NHCs, saturated in the backbone of the five membered
ring, are available from aliphatic 1,2-diamino derivatives, 1-(4-
nitropenyl)-ethan-1,2-diamine (3) was the crucial intermediate in
this synthesis (Scheme 1). Reduction of the corresponding diimine
2 turned out to be the best way to obtain this compound. Com-
pound 2 is available from 4-nitrophenylglyoxal (1). For the syn-
thesis of 1, we applied the oxidation of 4-nitroacetophenon with
SeO2 [13]. Although using a quite expensive and toxic oxidizing
agent, this reaction provides a series of advantages: Most impor-
tant, the product 1 is accessible in good yields and in excellent
purity. The use of acetic acid as the solvent is crucial for the success
of this reaction [14].
The dicarbonyl compound 1 was obtained as the bright yellow
colored oily hydrate, from which the water was split off thermally
by distillation leading to the intensely yellow colored solid dicar-
bonyl derivative 1. NMR spectroscopic data confirm the formation
of 1: Beside two doublets in the aromatic region, the 1H NMR
spectrum shows a singlet at 9.65 ppm, typical for an aldehyde
proton. In the 13C NMR spectrum, there are two resonances for the
carbonyl groups at 188.4 (CH]O) and 185.9 ppm (C]O). Reacting 1
with two equivalents of mesitylamine in formic acid gave the dii-
mine 2 in excellent yields [15]. A comparison of the NMR data of
compounds 1 and 2 confirms the formation of 2: The resonance
9.65 ppm had disappeared. From the 1H and the 13C NMR spectrum
of compound 2 it becomes evident, that there are two isomeric
Scheme 1. Synthesis of the 4-nitrophenyl functionalized imidazolinium salts 4aec.
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F. Rajabi et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
3
hindered for at least one of the mesityl groups. In the 13C NMR
spectrum, there are some broadened peaks too, supporting this
hypothesis. To further investigate this phenomenon, variable
temperature 1H NMR experiments were carried out with com-
pound 4a (Fig. 1). At room temperature (298 K), there are two quite
sharp signals (2.35 and 2.24 ppm), which we assign to the mesityl
methyl groups in the para-position and two broad signals (2.54 and
1.81 ppm) in the aliphatic region. Their integrals do not fit with the
expected 18 protons belonging to six methyl units. However,
cooling the sample to 238 K clearly resulted in six dominant sin-
glets for six different methyl groups. A group of two ortho-methyl
groups with resonances at around 2.50 ppm is close to the point of
coalescence at 298 K, the fourth ortho-methyl unit is strongly
shielded. Its resonance appears at around 1.77 ppm (at 238 K),
indicating a position close to the nitrophenyl unit. The dynamics of
the mesityl units is not the same: for one pair of methyl groups, the
point of coalescence is found at about room temperature, while the
other pair undergoes coalescence at about 350 K. Besides the
typical seven-line pattern of the solvent CD3CN (1.94 ppm), there
are two small singlets: One appearing at 2.08 (298 K) and slightly
becoming broader at higher temperatures, which we assign to re-
sidual water from the solvent. The second one appears as a sharp
signal at 2.47 ppm at 238 K (ꢀ60 ꢁC). It strongly shifts to lower field,
becomes broader by increasing the temperature and vanishes at
about 298 K. We assign this resonance to water molecules inter-
acting with the imidazolinium ring at low temperatures. The NMR
data of all compounds are deposited in the Supporting Information
belonging to this manuscript.
Single crystals suitable for an X-ray diffraction analysis could be
obtained by recrystallization of compound 4b from methanol. Fig. 2
shows the molecular structure of 4b in the solid state containing a
disordered tetrafluoroborate anion and selected bond parameters.
In the solid state structure of 4b, the counteranion BFꢀ4 is found
to be statistically disordered over two positions and does not un-
dergo hydrogen bonding to CH unit in the 2-position of the imi-
dazolinium ring. This site is perfectly shielded by the two N-mesityl
moieties, both of which and the nitrophenyl group are oriented
almost perpendicular to the imidiazolinium ring. The imidiazoli-
nium ring itself is only slightly bent. From this structure, the special
Fig. 2. Molecular structure of the imidazolinium salt 4b in the solid state. Selected
ꢁ
ꢀ
ꢀ
bond lengths [A], hydrogen bonds [A] and torsion angles [ ]: F1eB1A 1.393(11), F1eB1
1.378(6), F2eB1 1.367(6), F2eB1A 1.377(11), F3eB1 1.380(6), F3AeB1A 1.370(12), F4e
B1 1.369(7), F4AeB1A 1.373(12), N1eC1 1.477(2), N2eC2 1.501(2), N1eC3 1.307(2),
N2eC3 1.316(2), C1eC2 1.543(2), C3eH3 0.9500, H3/F1 2.3000, C3/F1 2.9365(18),
C3eN1eC19eC20 86.1(2), C1eN1eC3eN2 ꢀ2.78(19), C3eN2eC10eC11e89.8(2), C10e
N2eC2eC4 ꢀ7.25(19), N2eC2eC4eC9 114.34(17).
dynamics (see Fig. 1) of the imidazolinium salt 4a becomes clear:
due to the presence of the nitrophenyl group the free rotation of the
mesityl group attached to N2 is stronger hindered than the rotation
of the mesityl group attached to N2.
Reacting compound 4a with PdCl2 and K2CO3 in pyridine gave
the mono-NHC palladium complex 5 in 40% yield (Scheme 2). This
type of palladium(II) NHC complexes has recently attracted atten-
tion due to it’s high activities in different coupling reactions,
especially investigated by the group of Organ et al. [3,19].
The PdeNHC complex 5 is unambiguously identified by the
typical resonance of the carbene carbon atom in the 13C NMR
spectrum appearing at 184.8 ppm. In the 1H NMR spectrum, the
resonances of the coordinated pyridine ligand appear at 8.35
(o-pyr), 7.43 (p-pyr) and at approx. 7.00 ppp (m-pyr), showing only
Fig. 1. Temperature dependent 1H NMR data of compound 4a (in CD3CN, 1.94 ppm).
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4
F. Rajabi et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
Scheme 2. Synthesis of the PdeNHC complex 5.
a slight shift to lower field compared to free pyridine. This is due to
the strong trans-influence of the -donating NHC ligand. Parallely
s
the resonances of the protons at the NHC backbone are strongly
shifted toward higher field compared to the imidazolinium pre-
cursor 4a. While the resonances of the aromatic mesityl protons
overlap, the mesityl methyl groups give six well separated singlets,
proving a strong hindrance of the rotation around the N-mesityl
bonds. This is for sure not due to an electronic but a steric effect.
The resonance of one of the methyl units in the ortho-position is
found shifted to higher field, as it was the case for the
imidazolinium precursors 4aec (also see Fig. 1), implying a pro-
nounced shielding effect of the neighboring nitrophenyl moiety.
The hindered rotation of the mesityl groups leads to overall 27
resonances in the 13C NMR spectrum. Recrystallization of com-
pound 5 from a 1:1 mixture of DMSO/CH3CN by slow diffusion of
diethylether gave single crystals suitable for an X-ray diffraction
analysis. Fig. 3 shows the molecular structure of 5 in the solid state
and selected bond parameters. The structural parameters are in
complete agreement with the data published in the literature for
similar complexes [19].
Fig. 3. Molecular structure of the palladium complex 5 in the solid state, one co-
crystallized molecule of diethylether is omitted for clarity. Selected bond lengths [A],
ꢀ
bond angles [ꢁ] and torsion angles [ꢁ]: Pd1eCl1 2.3177(7), Pd1eCl2 2.2945(7), Pd1eN4
2.116(2), Pd1eC10 1.963(2), N1eC21 1.483(3), N1eC10 1.336(3), N2eC20 1.495(3), N2e
C10 1.339(3), C20eC21 1.527(3), Cl1ePd1eCl2 178.73(3), Cl1ePd1eN4 91.02(6), Cl1e
Pd1eC10 87.19(6), Cl2ePd1eN4 89.36(6), C10eN1eC21eC20 12.5(2), Cl2ePd1eN4e
C28 37.7(2), Cl1ePd1eC10eN1 79.06(19), C10eN1eC6eC5 77.1(3), C20eN2eC11eC12
79.8(3), N2eC20eC22eC23e118.2(2).
Fig. 3 clearly shows the perfect shielding of the palladium site by
the two N-mesityl moieties, resulting in an arrangement of the
Cl1eCl2 vector almost perpendicular to the imidiazolinium ring.
Slight steric repulsion of the protons located in the ortho-position of
the pyridine ring leads to a dihedral angle of approx. 37ꢁ between
the PdCl2 fragment and the pyridine ligand.
As expected, compound 5 is an highly active catalyst for the
SuzukieMiyaura cross coupling reaction. With only 0.05 mol-% of 5
the coupling of phenylboronic acid and a series of bromo arenes is
finished after approx. 20 min at room temperature (Table 1).
13C NMR spectra were recorded on Bruker Avance 200 MHz, DPX
400 MHz and Avance 600 MHz spectrometers at 25 ꢁC. Chemical
shifts are referenced internally to residual solvent signals. The
measurements of the crystal X-ray structures were carried out on
an Oxford Diffraction Gemini S Ultra diffractometer. All elemental
analyses were obtained from the microanalytical laboratory of the
Fachbereich Chemie in Kaiserslautern.
Table 1
Results of the SuzukieMiyaura cross coupling reaction with compound 5 as the
catalyst.
3. Summary
Entry
AreBr
Product
Time [min] Yielda
We have successfully synthesized and fully characterized a
bulky NHCePd(II), complex possessing a nitrophenyl substituent in
the backbone of the saturated NHC ring system and an additionally
coordinated pyridine molecule. Studies focusing on the trans-
formation of the nitro group into other functional groups are on the
way. This will provide anchoring units for immobilizing this
electron-rich carbene ligand on the surface of organic and inorganic
supports. In general, this route should offer a common access to
asymmetrically functionalized NHC ligands. Furthermore, we are
testing the application of this ligand system in cross coupling and
olefin metathesis reactions.
1
15
20
20
99
94
92
2
3
4
20
20
93
90
4. Experimental
5
4.1. General remarks
a
5 mmol of the aryl halide, 5.1 mmol of phenylboronic acid, 5 mL of isopropanol,
1.7 mg (0.05 mol-%) of compound 5, 5 mL of a 1 M aqueous solution of Na2CO3,
isolated yields.
All reagents and solvents were purchased from common
commercial sources used without further purification. The 1H and
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F. Rajabi et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
5
4.2. 4-Nitrophenylglyoxal (1)
128.9, 124.8 (12 ꢂ CAr), 62.6, 54.6 (NCH2, NCH), 21.7, 21.6, 20.2, 19.3
(4 ꢂ C6H2(CH3)3).
13.46 g (121.4 mmol) of selenium dioxide are added to 60 mL of
acetic acid and 9 mL of water. The mixture was stirred at 60 ꢁC until
solid had dissolved. Then the reaction mixture was heated to 115 ꢁC,
20.00 g (121.2 mmol) of 4-nitroacetophenone were added and the
heating was continued for further 3 h until the solution became
clear and the precipitation of metallic selenium stopped. To remove
the selenium precipitate, the solution was filtered while hot. The
solvents were stripped off leading to a bright yellow viscous oil,
mainly consisting of the hydrate of compound 1. By distillation at
1.3 ꢂ 10ꢀ1 mbar and 112 ꢁC the hydrate decomposed to water and
the intensely yellow colored product. The distillation column was
gently heated from time to time to liquefy the material (yield: 65%).
4.5. 4-(4-Nitrophenyl)-1,3-bis-(2,4,6-trimethylphenyl)-4,5-
dihydro-3H-imidazolium chloride (4a)
A 25 mL round bottom flask equipped with a reflux condenser
was charged with 3.53 g (8.5 mmol) of 3, 0.66 g (12.3 mmol) of
NH4Cl and 10.00 ml (60.7 mmol) of triethylorthoformate. The
mixture was heated for 8 h to 120 ꢁC, then cooled to r.t. and treated
with 20 mL of diethyl ether. Filtering off the white precipitate,
washing it with 10 mL of diethyl ether and drying it under oil
pump vacuum resulted in 3.25
g
of 4a (yield: 82%).
C
27H30N3Cl1O2$(NH4Cl)0.9: calcd. C 63.32, H 6.61, N 10.67; found C
1H NMR (600 MHz, CDCl3):
d
9.65 (s, 1H, CHO), 8.39 (d,
63.19, H 6.50, N 10.71. IR (KBr, cmꢀ1):
y
¼ 3215, 3078, 2965, 2860,
~
3
3JHH ¼ 8.9 Hz, 2H), 8.34 (d, JHH ¼ 8.9 Hz, 2H). 13C NMR (150 MHz,
2457, 2066, 1925, 1717, 1686, 1624, 1521, 1482, 1459, 1383, 1349,
CDCl3):
d
188.4 (CHO), 185.9 (CO), 151.2 (C-p ¼ CNO2), 136.1 (C-i),
1256, 1214, 1109, 1036, 1014, 859, 751, 734, 698, 573. 1H NMR
131.6 (C-m), 124.0 (C-o).
(400 MHz, CDCl3):
d
9.43 (s, 1H, NCHN), 8.09, 7.53 (2 ꢂ d,
3JHH ¼ 8.7 Hz, 2 ꢂ 2H, C6H4NO2), 6.91e6.85 (3 ꢂ br, 3H, C6H2(CH3)3),
6.61 (br, 1H, C6H2(CH3)3), 6.49 (dd, J ¼ 12.2, 9.6 Hz,1H, NCH), 5.20 (t,
J ¼ 12.6 Hz, 1H, NCH2), 4.40 (dd, J ¼ 12.6, 9.5 Hz, 1H, NCH2), 2.48 (br,
3H, C6H2(CH3)3), 2.45 (br, 3H, C6H2(CH3)3), 2.38 (br, 3H,
C6H2(CH3)3), 2.22 (s, 3H, C6H2(CH3)3), 2.12 (s, 3H, C6H2(CH3)3), 1.76
4.3. N,N0-(1-(4-Nitrophenyl)ethan-1,2-diylidene,is(2,4,6-
trimethylanilin) (2)
13.21 g (73.7 mmol) of 4-nitrophenylglyoxal were dissolved in
80 mL of methanol and 2.00 mL of formic acid were added to the
reaction mixture. After the addition of 20.80 mL (148.1 mmol) of
2,4,6-trimethylaniline, the reaction mixture was stirred for 20 h at
r.t.. The resulting orange colored solid was filtered off and was
washed twice with 15 mL of methanol. The product was dried
under vacuum (yield: 80%). C26H27N3O2: calcd. C 75.52, H 6.58, N
~
(br, 3H, C6H2(CH3)3). 13C NMR (100 MHz, CDCl3):
d 159.5 (NCN),
157.0 (CNO2), 148.6, 140.8, 140.5, 140.4, 135.1, 130.5, 130.3, 130.1,
130.0, 128.5, 124.2, 66.1, 57.0 (NCH2, NCH), 21.1, 21.0, 20.0, 18.4
(4 ꢂ C6H2(CH3)3).
4.6. 4-(4-Nitrophenyl)-1,3-bis-(2,4,6-trimethylphenyl)-4,5-
dihydro-3H-imidazolium tetrafluoroborate (4b)
10.16; found C 75.28, H 6.91, N 9.50. IR (KBr, cmꢀ1):
y
¼ 3436, 2915,
2854, 1626, 1584, 1515, 1476, 1377, 1306, 1249, 1208, 1147, 1107, 856,
819, 693, 558. 1H NMR (400 MHz, CDCl3):
d
8.39 (s, C6H4NO2, major
A 25 mL round bottom flask equipped with a reflux condenser
was charged with 3.53 g (8.5 mmol) of 3, 1.49 g (9.1 mmol) of
NH4BF4 and, and 8.00 mL (48.7 mmol) of triethylorthoformate. The
mixture was heated to 120 ꢁC for 4 h. It then was cooled to room
temperature and treated with 20 mL of hot anhydrous ethanol.
Filtering off the white precipitate, washing it with 10 mL of ethanol
and drying it under oil pump vacuum gave 2.44 g of 4b (yield: 50%).
3
isomer), 8.34 (s, CH]N, minor isomer), 8.13 (d, JHH ¼ 8 Hz,
C6H4NO2, minor isomer), 7.86 (s, CH]N, major isomer), 7.42 (d,
C6H4NO2, minor isomer), 6.92, 6.88, 6.85, 6.78 (4 ꢂ s, C6H2(CH3)3,
both isomers), 2.30, 2.28, 2.25, 2.24, 2.17, 2.09, 2.01, 2.00 (8 ꢂ s,
C6H2(CH3)3, both isomers). 13C NMR (100 MHz, CDCl3):
d 163.8
(CH]N, major isomer), 163.0 (C]N, major isomer), 156.4 (C-
p ¼ CNO2), 149.0, 147.2, 144.4, 142.1, 134.8, 133.6, 130.5, 129.8, 129.1,
129.1, 128.8, 126.6, 124.7, 123.2, 122.8, 20.69, 20.7, 18.4, 18.2.
C
27H30N3O2B1F4: calcd. C 62.93, H 5.87, N 8.15; found C 62.02, H
5.81, N 8.15. IR (KBr, cmꢀ1):
¼ 3421, 3095, 2960, 2925, 2861, 1645,
~
y
1525, 1478, 1385, 1350, 1281, 1255, 1208, 1100, 963, 853, 769, 752,
4.4. N,N0-dimesityl-1-(4-nitrophenyl)ethan-1,2-diamine (3)
734, 698, 575, 521, 448. 1H NMR (400 MHz, CD3CN):
d
8.36 (s, 1H,
3
NCHN), 8.24, 7.77 (2 ꢂ d, JHH ¼ 6.9 Hz, 2 ꢂ 2H, C6H4NO), 7.17 (br,
2H, C6H2(CH3)3), 7.11 (br. 1H, C6H2(CH3)3), 6.85 (br, 1H, C6H2(CH3)3),
6.14 (dd, J ¼ 12.3, 9.8 Hz, 1H, NCH), 4.96 (t, J ¼ 12.8 Hz, 1H, NCH2),
4.75 (dd, J ¼ 13.2, 9.8 Hz, 1H, NCH2), 2.57 (br, 6H, C6H2(CH3)3), 2.47
(br, 3H, C6H2(CH3)3), 2.37 (s, 3H, C6H2(CH3)3), 2.26 (s, 3H,
C6H2(CH3)3), 1.83 (br, 3H, C6H2(CH3)3). 13C NMR (101 MHz, CD3CN):
10 g (24 mmol) of 2 were suspended in 250 mL of methanol and
mixture was cooled to 0 ꢁC. Then 12.2 g (194 mmol) sodiumcya-
noborohydride were added in one portion. After the addition of the
reducing agent, the pH of the solution became slightly alkaline
(pH w 8). It was adjusted to about pH ¼ 3 and kept at this value for
30 min by dropwise addition of conc. HCl. Then the solution was
brought to r.t. and further stirred for 1 h. Subsequently, a 2 M so-
lution of KOH in water was added to the mixture until a pH of 8e9 is
reached. 50 mL of water were added and the mixture was extracted
three times with 50 mL of diethyl ether. The organic phase was
washed with 20 mL of a saturated NaCl solution and subsequently
dried over MgSO4. The solvent was stripped off resulting in a bright
yellow solid (yield: 85%). C26H31N3O2: calcd. C 74.79, H 7.48, N
~
d
158.9 (NCN þ CNO2), 148.8, 140.8, 140.5, 139.6, 135.5, 130.1, 129.9,
129.6, 129.4, 128.3, 123.9, 65.7, 55.9 (NCH2, NCH), 19.8, 19.7, 17.5, 17.1
(4 ꢂ C6H2(CH3)3).
4.7. 4-(4-Nitrophenyl)-1,3-bis-(2,4,6-trimethylphenyl)-4,5-
dihydro-3H-imidazolium hexafluorophosphate (4c)
A 25 mL round bottom flask equipped with a reflux condenser
was charged with 3.53 g (8.5 mmol) of 3, and 1.47 g (9.0 mmol) of
NH4PF6, and 10.00 mL (60.7 mmol) of triethylorthoformiate. The
mixture was heated to 120 ꢁC for 6 h. It then was cooled to room
temperature and treated with 20 mL of diethyl ether. Filtering off
the white precipitate, washing it with 10 mL of ethanol and drying
it under oil pump vacuum gave 3.79 g of 4c (yield: 78%).
10.06; found C 74.97, H 7.43, N 10.03. IR (KBr, cmꢀ1):
y
¼ 3375, 2916,
2855, 2730, 2366, 1732, 1605, 1521, 1483, 1446, 1345, 1228, 1107,
1012, 855, 700, 563. 1H NMR (400 MHz, CDCl3):
d
8.30, 7.58 (2 ꢂ d,
3JHH ¼ 8.7 Hz, 2 ꢂ 2H, C6H4NO2), 6.97, 6.95 (2 ꢂ s, 2 ꢂ 2H,
3
3
C6H2(CH3)3), 4.77 (dd, JHH ¼ 4.0 Hz, JHH ¼ 8.0 Hz, 1H, NCH), 3.68
2
3
(dd, JHH ¼ 12.0 Hz, JHH ¼ 4.0 Hz, 1H, NCH2), 3.56 (dd, J ¼ 12.0,
8.0 Hz, 1H, NCH2), 2.42 (s, 3H, C6H2(CH3)3), 2.39 (s, 3H, C6H2(CH3)3),
C
27H30N3O2P1F6,(NH4PF6)0.05: calcd. C 55.75, H 5.23, N 7.34; found
2.35 (s, 12H, C6H2(CH3)3). 13C NMR (100 MHz, CDCl3):
d
151.1 (C-
C 55.58, H 5.05, N 7.14. IR (KBr, cmꢀ1):
y
¼ 3666, 3333, 3088, 2926,
~
p ¼ CNO2), 148.3, 143.7, 142.3, 132.9, 132.2, 131.0, 130.9, 130.7, 129.7,
2865, 1631, 1529, 1479, 1434, 1350, 1250, 1211, 1062, 842, 734, 699,
Please cite this article in press as: F. Rajabi, et al., Journal of Organometallic Chemistry (2013), http://dx.doi.org/10.1016/
j.jorganchem.2013.05.025

6
F. Rajabi et al. / Journal of Organometallic Chemistry xxx (2013) 1e7
558, 530, 521. 1H NMR (400 MHz, DMSO):
d 9.23 (s, 1H), 8.29, 7.80
Table 2
(2 ꢂ 2d, 3JHH ¼ 8.8 Hz, 2 ꢂ 2H), 7.18, 7.17, 7.10, 6.85 (4 ꢂ br, 4H), 6.24
(dd, J ¼ 11.9, 9.9 Hz, 1H, NCH), 4.98 (t, J ¼ 12.5 Hz, 1H, NCH2), 4.87
(dd, J ¼ 12.7, 9.8 Hz, 1H, NCH2), 2.53, 2.52, 2.52, 2.46, 2.33, 2.21, 1.79
Crystallographic data, data collection and refinement.
4b
5
Empirical formula
Formula weight
Crystal size [mm]
C27H30BF4N3O2
515.35
C34H39Cl2N4O2.5Pd
720.99
(6 ꢂ s, 18H, C6H2(CH3)3). 13C NMR (101 MHz, DMSO):
d 160.0, 148.4,
0.19 ꢂ 0.06 ꢂ 0.04
0.30 ꢂ 0.17 ꢂ 0.04
140.4, 140.0, 139.6, 135.6, 135.5, 135.3, 130.7, 130.6, 130.0, 129.6,
129.4, 129.0, 124.0, 65.2, 55.6 (NCH2, NCH), 20.5, 20.4, 18.117.9, 17.6,
T [K]
150(2)
150(2)
ꢀ
l
[A]
1.54184
monoclinic
P21/n
15.6317(3)
9.4610(2)
17.6726(3)
90
100.757(2)
90
2567.70(9)
4
1.333
0.874
3.47e62.71
17,704
1.54184
orthorhombic
P21212
14.8943(1)
23.5103(2)
10.2095(1)
90
17.3 (4 ꢂ C6H2(CH3)3). 31P NMR (162 MHz, DMSO):
d 144.19 (hept,
Crystal system
Space group
1JPF ¼ 711.3 Hz).
ꢀ
a [A]
ꢀ
b [A]
4.8. Dichlorido(pyridine)[4-(4-nitrophenyl)-1,3-bis-(2,4,6-
trimethylphenyl)-4,5-dihydro-3H-imidazol-2-ylidene]palladium(II) (5)
ꢀ
c [A]
a
b
g
[ꢁ]
[ꢁ]
[ꢁ]
90
90
Amixture of0.548g(1.18mmol)of4a, 0.105g(0.59mmol)ofPdCl2
and 0.326 g (2.36 mmol) of K2CO3 was stirred in 6 mL of pyridine for
3 d at 60 ꢁC under an atmosphere of N2. After cooling the reaction
mixture to r.t., most of the volatiles were removed under reduced
pressure. The solid residue was purified via flash chromatography
(hexane:ethyl acetate, 5:1) to give the NHCePd(II) complex 5 as a
yellow solid (yield: 40%). The single crystal used for X-ray diffraction
was obtained by recrystallization from DMSO:CH3CN (1:1) and
3
ꢀ
V [A ]
3575.05(5)
4
1.340
5.842
3.51e62.70
27,303
5711 [Rint ¼ 0.0319]
5711/0/401
0.0198, 0.0521
0.0205, 0.0524
1.047
0.215(5)
0.232/ꢀ0.252
Z
rcalcd. [g cmꢀ3
]
m
[mmꢀ1
]
q
erange [ꢁ]
Refl. coll.
Indep. refl.
Data/restr./param.
Final R indices [I > 2
R indices (all data)
GooFb
4099 [Rint ¼ 0.0272]
4099/149/368
0.0403, 0.1146
0.0484, 0.1185
1.072
s
(I)]a
diffusion of diethylether.
C32H35Cl2N4O2Pd$(CH3CN)$(C2H6SO)0.5:
calcd. C 55.97, H 5.50, N 7.46; found C 55.64, H 5.50, N 7.77. IR (KBr,
cmꢀ1):
y
¼ 3666, 3334, 3088, 2927, 2865,1631,1530,1479,1434,1350,
~
Flack parameter
Drmax
e
ꢀꢀ3
1250, 1211, 1062, 842, 734, 699, 558, 530, 521. 1H NMR (200 MHz,
/
(e$A
)
0.0309/ꢀ0.277
P
min
P
P
P
8.35 (dd, 3JHH ¼ 6.5, 4JHH ¼ 1.5Hz, 2H, o-pyr), 8.14, 7.46 (2ꢂ d,
a
CDCl3):
d
R1 ¼ jjFoj ꢀ j:Fcj:j= jFoj;
Good ¼ ½
u
R2 ¼ ½
u
ðFo2 ꢀ Fc2Þ2=
u
Fo2ꢃ1=2
:
3JHH ¼ 8.7 Hz, 2 ꢂ 2H, C6H4NO), 7.43 (m,1H, p-pyr), 7.05e6.95 (m, 4H,
m-pyr, C6H2(CH3)3), 6.89 (br, 1H, C6H2(CH3)3), 6.70 (br, 1H,
C6H2(CH3)3), 5.32 (dd, 3JHH ¼ 11.8, 3JHH ¼ 7.1 Hz, 1H, NCH), 4.40 (dd,
2JHH ¼ 3JHH ¼ 11.8 Hz, 1H, NCH2), 4.14 (dd, 1H, NCH2), 2.67, 2.61, 2.54,
2.28, 2.18, 2.02 (6 ꢂ s, 6 ꢂ 3H, C6H2(CH3)3). 13C NMR (50 MHz, CDCl3):
P
b
u
ðFo2 ꢀ Fc2Þ2=ðn ꢀ pÞꢃ1=2
:
occupied and severely disordered solvent molecules, which could
not be assigned (any small molecule may be possible). Hence the
SQUEEZE process integrated in PLATON was used and detailed in-
formation were posted in the final CIF file. The crystal studied was a
racemic twin.
d
184.8 (NCNePd), 151.0 (CNO2),148.0,144.2,138.6,138.2,138.1,137.2,
137.1, 136.9, 136.4, 134.2, 133.3, 129.7, 129.6, 129.6, 129.4, 123.8,
123.7, 66.1 (NCH), 57.4 (NCH2), 20.8, 20.8, 20.0, 19.9, 19.8, 19.3
(6 ꢂ C6H2(CH3)3).
Acknowledgments
4.9. General procedure for the SuzukieMiyaura cross coupling
This work was supported by the DFG funded transregional
collaborative research center 3MET. F.R. gratefully acknowledges
the Alexander von Humboldt-Foundation for a research grant.
reactions
In a typical run, a 50 mL round bottom flask equipped with a
magnetic stirring bar was charged with the aryl halide (5 mmol),
phenylboronic acid (5.1 mmol), and 5 mL of isopropanol. After the
compounds had completely dissolved 1.7 mg (0.05 mol-%) of
complex 5 and 5 mL of an 1 M aqueous Na2CO3 solution were was
added. After the appropriate time given in Table 1 20 mL of water
were poured to the reaction mixture, the organic layer was
extracted with ethyl acetate and was dried over Na2SO4. The so-
lution was filtered over a silica and the solvent was removed by
evaporation to give an almost pure product.
Appendix A. Supplementary material
CCDC-940508 (4b) and CCDC-940509 (5) contain the supple-
mentary crystallographic data belonging to this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Appendix B. Supplementary material
4.10. X-ray structure analyses
Supplementary material related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2013.05.025.
Crystal data and refinement parameters for compounds 4b and
5 are collected in Table 2. The structures were solved using direct
methods (SIR92 [20]), completed by subsequent difference Fourier
syntheses, and refined by full-matrix least-squares procedures [21].
Semi-empirical absorption corrections from equivalents (Multi-
scan) were carried out [22]. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were placed in calculated positions and refined by using a riding
model. For compound 4b, the counteranion BFꢀ4 was disordered. In
the solid structure of compound 5, one molecule was co-
crystallized with 0.5 equivalents of diethylether. The examination
of the relatively high residual electron densities in the final differ-
ence Fourier synthesis indicates the existence of some partially
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j.jorganchem.2013.05.025