Methoxyaryl substituted palladium bis-NHC complexes - Synthesis and electronic effects

Article abstract of DOI:10.1016/j.ica.2012.06.009

A series of methoxyphenyl substituted chelated bis-N-heterocyclic carbene palladium(II) complexes has been synthesized and characterized by cyclovoltammetry, spectroscopy (NMR, IR), solid state structures and investigated by quantum chemical calculations.

Full text of DOI:10.1016/j.ica.2012.06.009

Inorganica Chimica Acta 392 (2012) 204–210
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Methoxyaryl substituted palladium bis-NHC complexes – Synthesis and
electronic effects
Dominik Munz a, Christoph Allolio a, Katrin Döring b, Alexander Poethig a,1, Thomas Doert c,1
,
Heinrich Lang b, Thomas Straßner a,
⇑
a Physikalische Organische Chemie, Technische Universität Dresden, Bergstrasse 66, 01062 Dresden, Germany
b Institut für Chemie, Professur Anorganische Chemie, Technische Universität Chemnitz, Straße der Nationen 62, 09111 Chemnitz, Germany
c Anorganische Chemie, Technische Universität Dresden, Bergstrasse 66, 01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of methoxyphenyl substituted chelated bis-N-heterocyclic carbene palladium(II) complexes has
been synthesized and characterized by cyclovoltammetry, spectroscopy (NMR, IR), solid state structures
and investigated by quantum chemical calculations. The results confirm that a methoxy functionality in
para position at the aryl substituent enhances the donor properties of the ligand compared to a methoxy
group in ortho or meta position.
Received 10 January 2012
Received in revised form 24 April 2012
Accepted 7 June 2012
Available online 21 June 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Palladium(II)–NHC-complexes
Biscarbene
Solid state structure
Electronic effects
NHC
Cyclovoltammetry
1. Introduction
show that the catalytic activity of aryl substituted, chelated imi-
dazoline-2-ylidene palladium and platinum complexes can be
The discovery of a stable N-heterocyclic carbene (NHC) by Ardu-
engo [1] in 1991 led to a new and rapidly advancing area of re-
search. A large number of publications on NHC metal complexes
has been published and reviewed in recent years [2–9]. Properties
of NHC complexes include a high thermal stability as the ligands
generally do not dissociate easily from the metal. There is also evi-
tuned by electron donating and withdrawing groups in para posi-
tion on the aryl ring. Examples include the Heck reaction and the
C–H activation of methane [25,26]. Scheme 1 shows the three dif-
ferent methoxyphenyl substituted bis-NHC palladium(II) com-
plexes studied in this work.
The para methoxy substituted complex 3 was found to be a con-
siderably more efficient catalyst than other para functionalized
aryl substituted bis-NHC palladium complexes (i.e., the analogous
para bromophenyl or para nitrophenyl complexes) in the Heck
reaction of aryl halogenides with styrene [25]. In the CH activation
of methane we observed that electron donating substituted aryl
bis-NHC complexes like for example 3 lead to lower turnover num-
bers than electron withdrawing substituted ones [27]. Therefore
we decided to study the effect of the position of the activating
methoxy groups in detail and synthesized new complexes where
we varied their position on the aryl ring in order to further eluci-
date its effect on the electronic properties.
dence indicating that carbene ligands are stronger
p-donors than
many phosphine based systems [10,11]. The catalytic activity of
organometallic complexes is often linked to the donor properties
of the ligands [12,13] and we therefore have been interested in
finding ways to tune them. While in the past NHC ligands were as-
sumed to be pure
r-donors, we believe that significant backbond-
ing via the -orbitals of the heterocycle has an effect on the donor
p
properties [14,15].
Chelated palladium bis-NHC complexes have been shown to
possess especially high stability against oxidizing and acidic condi-
tions, making them suitable e.g. for the C–H activation of methane
in trifluoroacetic acid [16,17]. They have also found wide applica-
tion in C,C coupling reactions [18–24]. Recently, we were able to
We expected differences for the resulting complexes based on
resonance considerations as well as Hammett
r-constants: a
methoxy group in meta position should not be able to act as
a donor through its mesomeric effect and should instead become
a net acceptor through the inductive effect of the electronegative
oxygen. This is reflected by a change of sign in the corresponding
⇑
Corresponding author. Tel.: +4935146338571.
E-mail address: thomas.strassner@chemie.tu-dresden.de (T. Straßner).
X-ray analysis.
1
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.06.009

D. Munz et al. / Inorganica Chimica Acta 392 (2012) 204–210
205
[Pd(OAc)2] were stirred in 5 mL DMSO at ambient temperature.
After 2 h the formation of a white precipitate was observed. After
12 h the solution was heated at 60 °C for 4 h. At the end of the reac-
tion the solvent was removed under reduced pressure and the
resulting solid was washed twice with 3 mL of MeOH, MeCN and
CH2Cl2. Volatiles were removed in vacuo to yield an off-white solid
(0.17 g, 69% based on [Pd(OAc)2]).
N
N
1
R = ortho-OCH3
2 R = meta-OCH3
R = para-OCH3
N
N
Pd
Br Br
R
R
3
Scheme 1. Methoxyphenyl substituted chelated bis-NHC palladium(II) complexes.
1H NMR (500.13 MHz, DMSO-d6, T = 323 K): d 7.78 (s, 2H, NCH);
7.68–7.42 (m, 6H, Ar and NCH); 7.33–7.08 (m, 4H, Ar); 6.50 (s, 2H,
NCH2N); 3.83 (s, 6H, CH3) ppm. 13C NMR (125.77 MHz, DMSO-d6,
T = 353 K): d 153.25 (i-C of Ar); 129.71 (CH); 128.06 (CH); 124.37
(CH); 120.70 (CH); 119.71 (CH); 112.68 (CH); 62.59 (CH2); 54.83
(CH3) ppm; carbene carbon atom signal not detected due to poor
solubility. Mp. > 300 °C. MS (ESI): m/z = 525.1 [PdL(OAc)]+, 547.0
[PdLBr]+. Anal. Calc. for C21H20N4Br2O2Pdꢁ0.8 C2H6SO: C, 39.39; H,
3.63; N, 8.13; S, 3.72. Found: C, 39.51; H, 3.51; N, 8.31; S, 3.75%.
Hammett parameter [28] and it should be possible to measure the
effect, if
p-donation from the aryl substituent has an influence on
the metal.
In order to characterize the electronic effects of the ligands we
employed different methods: cyclovoltammetry (CV), DFT calcula-
tions, NMR and IR spectroscopy. The direct experimental quantifi-
cation of donor properties and other electronic effects by CV has
already been proven to be a valuable tool for the characterization
of NHC complexes [29–32] and it was employed in the determina-
tion of the Lever electronic parameter (LEP) [33] for a wide variety
of common ligands. Also DFT calculations are widely applied for
the estimation of donor effects [34,35]. Another way to determine
the donor strength has been introduced by Tolman (TEP) [36]. In
order to study the effect of the position of the methoxy group,
we synthesized the new complexes 1 and 2, which are isomers of
the known complex 3 [25].
2.2.2. 3,30-Bis(3-methoxyphenyl)-[(1,10-diimidazolin-2,20-diylidene)
methane] palladium(II) dibromide (2)
200 mg (0.38 mmol) of 3,30-Bis-(3-methoxyphenyl)-1,10-meth-
ylene-diimidazolium-dibromide (6) and 80 mg (0.36 mmol)
[Pd(OAc)2] were heated in 5 mL DMSO from ambient temperature
to 60 °C for 18 h. After 2 h the formation of a white precipitate was
observed. At the end of the reaction the solvent was removed un-
der reduced pressure and the resulting white solid was washed
twice with 2 mL of MeOH, THF and CH2Cl2. Volatiles were removed
in vacuo to yield an off-white solid (0.20 g, 85% based on
[Pd(OAc)2]).
2. Experimental
2.1. Material and methods
1H NMR (500.13 MHz, DMSO-d6, T = 323 K): d 7.91–7.69 (m, 4H,
NCH); 7.66–7.23 (m, 6H, Ar); 7.06 (s, 2H, Ar); 6.53 (s, 2H, NCH2N);
3.93 (s, 6H, CH3) ppm. 13C NMR (150.91 MHz, DMSO-d6, 300 K): d
159.02 (i-C of Ar); 140.12 (i-C of Ar); 129.04 (CH of Ar); 122.43
(NCH), 121.87 (NCH), 116.64 (CH of Ar); 113.54 (CH of Ar);
110.83 (CH of Ar); 62.96 (CH2); 55.11 (CH3) ppm; carbene carbon
signal not detected due to poor solubility. Mp. > 300 °C. MS (ESI):
m/z = 525.1 [PdL(OAc)]+. Anal. Calc. for C21H20N4Br2O2Pdꢁ0.35
DMSO: C, 39.85; H, 3.41; N, 8.57; S, 1.72. Found: C, 39.56; H,
3.09; N, 8.56; S, 1.68%.
Solvents of at least 99.5% purity were used throughout this study.
All other chemicals were obtained from common suppliers and used
without further purification. THF and DMF were dried by standard
procedures prior to use. 3,30-Bis(4-methoxyphenyl)-[(1,10-diimi-
dazolin-2,20-diylidene)methane] palladium(II) dibromide 3 was
prepared according to published protocols [25]. Imidazoles 4 and
5 have been synthesized following a modified procedure which is
described in detail in the Supplementary material.
1H and 13C NMR spectra were recorded with a Bruker AC 300 and
Bruker DRX 500 P spectrometer. The spectra were referenced inter-
nally to the resonances of the solvent (1H, 13C). Elemental analyses
were performed by the microanalytical laboratory of our institute
using a EuroVektor Euro EA-3000 Elemental Analyzer. Melting
and decomposition points were determined with a Wagner&Munz
PolyTherm A melting point apparatus and are uncorrected. IR spec-
tra were recorded with a Thermo Nicolet Avatar 360 E.S.P. (ATR)
using a resolution of 1 cmꢀ1, 64 scans and an aperture of 100.0.
The CV experiments were conducted in a custom made glass cell
under an argon atmosphere using a Radiometer Analytical PGZ
100 VoltaLab potentiostat. The experiments were conducted at
25 °C using a platinum work and counter electrode. As reference
electrode a non-aqueous calomel electrode was used. Acetonitrile
with a concentration of 0.1 M TBA-PF6 was employed as the solvent.
The solvent and TBA-PF6 were carefully dried prior to use. All orga-
nometallic compounds were added to achieve a concentration of
c = 0.1 mmol. All cyclovoltammograms were measured at a scan
rate of 100 mV sꢀ1. Positive mode ESI-MS spectra for the synthe-
sized compounds were recorded on a Bruker Esquire MS with Ion
Trap Detector on samples dissolved in NH4OAc buffered methanol.
2.3. Synthesis of imidazolium salts
2.3.1. 3,30-Bis(2-methoxyphenyl)-[(1,10-diimidazolium)methane]
dibromide (6)
1.30 g (7.5 mmol) 1-(2-Methoxyphenyl)-imidazole (4) and
0.28 mL (0.70 g, 4.0 mmol) dibromomethane were dissolved in
5 mL of THF. The reaction mixture was heated in a pressure tube
at 130 °C for 48 h. The brown precipitate was filtrated and washed
with 10 mL of cold THF. The resulting brown solid was stirred for
six days in 20 mL of EtOAc. The remaining crude product was fil-
trated and washed with THF until it was colorless. The resulting
hygroscopic solid was dried in vacuo (0.70 g, 36% based on 4).
1H NMR (300.13 MHz, DMSO-d6, T = 295 K): d 10.12 (s, 2H,
NCHN); 8.39 (s, 2H, NCH); 8.23 (s, 2H, NCH); 7.68 (d, J = 7.4 Hz,
2H, Ar); 7.63 (t, J = 8.2 Hz, 2H, Ar); 7.42 (d, J = 8.2 Hz, 2H, CH of
Ar); 7.22 (t, 2H, J = 7.4 Hz, CH of Ar); 6.97 (s, 2H, NCH2N); 3.92 (s,
6H, CH3) ppm. 13C NMR (74.475 MHz, DMSO-d6, T = 295 K): d
151.74 (i-C of Ar); 138.73 (NCH); 131.89 (CH of Ar); 125.03 (CH
of Ar); 124.06 (CH of Ar); 122.96 (i-C of Ar); 121.92 (CH of Ar);
113.37 (CH of Ar); 107.97 (CH of Ar); 58.37 (CH2), 56.40 (CH3)
ppm. Mp. 235.9 °C (decomp.). Anal. Calc. for C21H20N4O2: C,
48.30; H, 4.25; N, 10.73. Found: C, 48.06; H, 4.04; N, 10.67%.
2.2. Synthesis of the complexes with bromide counterions
2.2.1. 3,30-Bis(2-methoxyphenyl)-[(1,10-diimidazolin-2,20-diylidene)
methane] palladium(II) dibromide (1)
2.3.2. 3,30-Bis(3-methoxyphenyl)-[(1,10-diimidazolium)methane]
dibromide (7)
1.80 g (10.3 mmol) of 1-(3-Methoxyphenyl)-imidazole (5)
were dissolved in 5 mL THF containing 0.36 mL (0.89 g, 5.2 mmol)
200 mg (0.38 mmol) of 3,30-Bis-(2-methoxyphenyl)-1,10-meth-
ylene-diimidazolium-dibromide (7) and 80 mg (0.36 mmol)

206
D. Munz et al. / Inorganica Chimica Acta 392 (2012) 204–210
dibromomethane. The reaction mixture was heated in a pressure
tube at 130 °C for 24 h. A white precipitate was formed. The crude
product was filtrated, washed with 10 mL of THF and the remain-
ing solvent was removed in vacuo to yield a colorless hygroscopic
solid (1.9 g, 71% based on 5).
110.95 (CH of Ar); 62.68 (CH2); 55.74 (CH3); 1.10 (CH3CN)
ppm. 19F NMR (282.40 MHz, DMSO-d6, T = 295 K): d ꢀ70.14 (d,
J = 711.2 Hz) ppm. Mp. 229.6 °C (decomp.). MS (ESI): m/z = 525.1
[PdL(OAc)]+. Anal. Calc. for C25H26F12N6O2P2Pd: C, 35.79; H, 3.12;
N, 10.02. Found: C, 35.60; H, 2.96; N, 9.94%.
1H NMR (300.13 MHz, DMSO-d6, T = 295 K): d 10.28 (s, 2H,
NCHN); 8.46 (t, J = 2.0 Hz,2H, CH of Ar); 8.33 (s, 2H, NCH); 7.62 (t,
2H, J = 8.3 Hz, CH of Ar); 7.44 (s, 1H, NCH); 7.38 (d, 2H J = 8.3 Hz,
CH of Ar); 7.21 (dd, 2H, J1 = 8.3 Hz, J2 = 2.0 Hz, CH of Ar); 6.85 (s,
2H, NCH2N); 3.88 (s, 6H, CH3) ppm. 13C NMR (74.475 MHz, DMSO-
d6, T = 295 K): d 160.37 (i-C of Ar, 8); 137.40 (NCHN, 1); 135.42 (i-C
of Ar); 131.36 (C of Ar); 122.96 (NCH); 121.66 (NCH); 115.70 (CH
of Ar); 113.71 (CH of Ar); 107.97 (CH of Ar); 55.94 (CH2), 49.23
(CH3) ppm. Mp. 323.2 °C (decomp.). Anal. Calc. for C21H20N4O2: C,
48.30; H, 4.25; N, 10.73. Found: C, 48.66; H, 4.07; N, 10.91%.
2.4.3. 3,30-Bis(4-methoxyphenyl)-[(1,10-diimidazolin-2,20-diylidene)
methane] (bis-acetonitrile) palladium(II) hexafluorophosphate (10)
80 mg (0.12 mmol) 3,30-Bis(4-methoxyphenyl)-[(1,10-diimidaz-
olin-2,20-diylidene)methane] palladium(II) dibromide (3) and
71 mg [AgPF6] (0.28 mmol) were suspended in 3 mL MeCN and
stirred at 60 °C for 9 h. The solution was filtrated over Celite and
the solvent was removed subsequently. The crude product was
washed twice with 2 mL H2O, dried in vacuo, recrystallized by con-
densing Et2O into a saturated solution in MeCN and filtrated. Vol-
atiles were removed in vacuo to yield a colorless crystalline solid
(25 mg, 23% based on 3).
2.4. Synthesis of complexes with hexafluorophosphate counterions
1H NMR (300.15 MHz, DMSO-d6, T = 295 K): d 7.95 (d, J = 2.0 Hz,
2H, NCH); 7.89 (d, J = 8.8 Hz, 4H, Ar); 7.85 (d, J = 2.0 Hz, 2H, NCH);
7.28 (d, J = 8.8 Hz, 4H, Ar); 6.62 (dd, J = 12.9 Hz, J = 50.4 Hz, 2H,
NCH2N); 3.88 (s, 6H, CH3); 2.07 (s, 6H, NCCH3) ppm. 13C NMR
(74.475 MHz, DMSO-d6, T = 295 K): d 159.71 (i-C of Ar); 146.32
(i-C, Carbene); 130.91 (i-C of Ar); 126.11 (CH of Ar); 123.73
(NCH); 123.03 (NCH); 118.02 (CH3CN); 114.61 (CH of Ar); 62.56
(CH2); 55.74 (CH3); 1.10 (CH3CN) ppm. 19F NMR (282.40 MHz,
2.4.1. 3,30-Bis(2-methoxyphenyl)-[(1,10-diimidazolin-2,20-diylidene)
methane](bis-acetonitrile) palladium(II) hexafluorophosphate (8)
60 mg (0.10 mmol) of 3,30-Bis(2-methoxyphenyl)-[(1,10-diimi-
dazolin-2,20-diylidene)methane] palladium(II) dibromide (1) and
53 mg of [AgPF6] (0.21 mmol) were suspended in 2 mL of MeCN
and stirred at 60 °C for 9 h. The solution was filtrated over Celite
and the solvent was removed subsequently. The crude product
was washed twice with 2 mL H2O, dried in vacuo, recrystallized
by condensing Et2O into a saturated solution in MeCN and filtrated.
Volatiles were removed in vacuo to yield a colorless crystalline so-
lid (35 mg, 44% based on 1).
DMSO-d6, T = 295 K):
d
ꢀ70.12 (d, J = 711.7 Hz) ppm. Mp.
223.9 °C (decomp.). MS (ESI): m/z = 525.1 [PdL(OAc)]+. Anal. Calc.
for C25H26F12N6O2P2Pd: C, 35.79; H, 3.12; N, 10.02. Found: C,
36.07; H, 3.05; N, 10.09%.
1H NMR (300.15 MHz, DMSO-d6, T = 295 K): d 7.89 (d, J = 1.9 Hz,
2H, NCH); 7.88 (d, J = 7.6 Hz 2H, Ar); 7.72 (d, J = 1.9 Hz, 2H, NCH);
7.60 (t, J = 7.6 Hz, 2H, Ar); 7.36 (d, J = 7.6 Hz, 2H, Ar); 7.25 (t,
J = 7.6 Hz, 2H, Ar); 6.61 (d, J = 26.6 Hz, 2H, NCH2N); 3.89 (s, 6H,
CH3); 2.07 (s, 6H, NCCH3) ppm. 13C NMR (74.475 MHz, DMSO-d6,
T = 295 K): d 153.15 (i-C of Ar, 12); 147.35 (i-C, Carbene); 130.92
(CH of Ar); 128.06 (CH of Ar); 126.57 (i-C of Ar); 125.12 (NCH);
122.29 (NCH); 120.52 (CH of Ar); 117.99 (CH3CN); 112.73 (CH of
Ar); 56.01 (CH3); 1.06 (CH3CN) ppm. 19F NMR (282.40 MHz,
An ORTEP plot of 10 in the solid state is shown in the Supplemen-
tary material (together with the corresponding crystallographic de-
tails). Thermal ellipsoids are drawn at 30% probability level showing
two independent molecules of 10 in the asymmetric unit.
2.5. Structure determination of compound 9 (8, 10 see Supp. material)
For solid state structure analysis preliminary examination and
data collection were carried out on a NONIUS
an Oxford Cryosystems cooling system at the window of a sealed
fine-focus X-ray tube with graphite-monochromated Mo K radia-
j-CCD device with
DMSO-d6, T = 295 K):
d
ꢀ70.14 (d, J = 711.2 Hz) ppm. Mp.
155.3 °C (decomp.). MS (ESI): m/z = 525.1 [PdL(OAc)]+. Anal. Calc.
for C25H26F12N6O2P2Pd: C, 35.79; H, 3.12; N, 10.02. Found: C,
35.84; H, 2.91; N, 9.66%.
a
tion (k = 0.71073 Å). The reflections were integrated, raw data were
corrected for Lorentz and polarization effects and, arising from the
scaling procedure, for latent decay. An absorption correction was
applied using SADABS [37]. After merging, the independent reflec-
tions were all used to refine the structure. The structure was solved
by a combination of direct methods [38] and difference Fourier
synthesis [39]. All non-hydrogen atom positions were refined with
anisotropic displacement parameters. All hydrogen atoms were
placed in calculated positions and afterwards refined using the
SHELXL-97 riding model. Full-matrix least-squares refinements
An ORTEP plot of 8 in the solid state is shown in the Supplemen-
tary material (together with the corresponding crystallographic
details). Thermal ellipsoids are drawn at 50% probability level
showing two independent molecules of 8 in the asymmetric unit.
2.4.2. 3,30-Bis(3-methoxyphenyl)-[(1,10-diimidazolin-2,20-diylidene)
methane] (bis-acetonitrile) palladium(II) hexafluorophosphate (9)
100 mg (0.16 mmol) 3,30-Bis(3-methoxyphenyl)-[(1,10-diimi-
dazolin-2,20-diylidene)methane] palladium(II) dibromide (2) and
89 mg [AgPF6] (0.35 mmol) were suspended in 3 mL MeCN and
stirred at 60 °C for 9 h. The solution was filtrated over Celite and
the solvent was removed subsequently. The crude product was
washed twice with 2 mL H2O, dried in vacuo, recrystallized by con-
densing Et2O into a saturated solution in MeCN and filtrated. Vol-
atiles were removed in vacuo to yield a colorless crystalline solid
(45 mg, 34% based on 2).
2
2 2
were carried out by minimizing
R
w(Fo ꢀ Fc
) with the SHELXL-97
weighting scheme and stopped at shift/err <0.001. Neutral-atom
scattering factors for all atoms and anomalous dispersion correc-
tions for the non-hydrogen atoms were taken from the Interna-
tional Tables for Crystallography [40]. All calculations were
performed with the programs COLLECT [41], DIRAX [42], EVALCCD [43],
SIR97 [38], SADABS [37], the SHELXL-97 package [39,44] and ORTEP-III
[45]. The crystallografic details are given in Table 1.
1H NMR (300.15 MHz, DMSO-d6, T = 295 K): d 7.99 (d, J = 2.0 Hz,
NCH); 7.92 (d, J = 2.0 Hz, 2H, NCH); 7.72 (t, J = 1.9 Hz, 2H, Ar); 7.57
(t, J = 7.9 Hz, 2H, Ar); 7.50 (d, J = 7.9 Hz, 2H, Ar); 7.20 (dd, J = 1.9,
J = 7.9 Hz, 2H, Ar); 6.66 (dd, J = 13.0, J = 46.6 Hz, 2H, NCH2N); 3.96
(s, 6H, CH3); 2.07 (s, 6H, NCCH3) ppm. 13C NMR (74.475 MHz,
DMSO-d6, T = 295 K): d 159.80 (i-C of Ar); 146.56 (i-C, Carbene);
138.96 (i-C of Ar, 5); 130.34 (CH of Ar); 123.64 (NCH); 123.15
(NCH); 118.06 (CH of Ar); 116.68 (CH of Ar); 114.98 (CH3CN);
2.6. Computational details
All optimizations have been performed with the GAUSSIAN 03
program package [46]. They were carried out using the BP86
gradient corrected density functional [47,48] and the 6-31G(d) ba-
sis set [49,50] for all non-metal atoms. The BP86 functional was
chosen as it is known to reproduce experimental vibrational

D. Munz et al. / Inorganica Chimica Acta 392 (2012) 204–210
207
Table 1
Crystallographic details.
N
N
N
N
Pd(OAc)2
N
N
Complex
9
N
N
DMSO, ΔT
Pd
Br Br
Formula
a (Å)
b (Å)
C25 H26 F12 N6 O2 P2 Pd
8.6060(13)
18.876(3)
20.4680(15)
90.000
106.675(8)
90.000
3185.1(7)
4
R
R
2 Br
6, 7
R
R
c (Å)
1 R = ortho-OCH3 (69%)
R = meta-OCH3 (85%)
a
(°)
2
b (°)
c
(°)
V (Å3)
Scheme 3. Synthesis of complexes 1 and 2.
Z
Formula weight
Space group (No.)
T (°C)
838.86
P 21/c(14)
ꢀ75(2)
During the preparation of complexes 1–3 the crude product
readily precipitated from hot DMSO as a colorless solid. The very
low solubility of 1–3 in all examined organic solvents may be a
limiting factor for the catalytic performance in the Heck reaction
[25] and is an obstacle for the spectroscopic analysis, but is has also
been reported for similar bis-NHC ligands incorporating aryl rings
[21]. To get 13C and 1H NMR spectra of 1–3 with a decent signal/
noise ratio we had to heat the probe to 70 °C. Therefore we decided
that for the evaluation of the donor properties we needed to ‘‘de-
sign’’ better soluble species – the dicationic acetonitrile complexes.
Cationic palladium complexes have very recently drawn consider-
able attention, as they have often been shown to lead to signifi-
cantly milder reaction conditions than complexes with stronger
coordinating counterions like halogenides or trifluoroacetates.
Examples include the Suzuki-Miyaura coupling or CH-activation
strategies [58,59].
k (Å)
0.71073
Dcalcd (g cmꢀ3
)
1.75
0.789
R1 = 0.0378,
wR2 = 0.0882
R1 = 0.0537
wR2 = 0.0968
l
(mmꢀ1
)
Final R indices [I > 2
r(I)]
R indices (all data)
frequencies very well [51]. All structures were fully optimized
without constraints. Harmonic force constants were calculated
for all geometries in order to verify them as ground states. In all
cases palladium was described using
Wadt(n + 1) ECP and basis set [52,53].
a decontracted Hay-
3. Results and discussion
3.2. Syntheses of the complexes with hexafluorophosphate counterions
3.1. Synthesis of the complexes with bromide counterions
The dicationic acetonitrile complexes with hexafluorophos-
phate counteranions (8–10) (Scheme 4) were synthesized by silver
mediated counterion exchange starting from 1–3. By heating them
with [AgPF6] in acetonitrile the bromides could be replaced by the
The para methoxy substituted complex 3 was prepared accord-
ing to literature procedures [25], whereas the new complexes 1
and 2 have been synthesized as described in Schemes 2 and 3.
The arylimidazoles 4 and 5 were prepared by an Ullmann type cou-
pling of imidazole and methoxyphenyl iodide in the presence of
catalytic amounts of copper(I) oxide and phenanthroline. Copper(I)
catalyzed arylhalogenide–N-heterocycle couplings have found
widespread use in organic synthesis, as they provide straightfor-
ward access to N-arylated imidazoles in usually high yields
[54,55]. Phenanthroline and Cu2O are a frequently employed li-
gand/copper(I) precatalyst combination [56,57]. The bisimidazoli-
um salts 6 and 7 could be obtained in moderate yields by
heating two equivalents of the corresponding methoxyphenyl imi-
dazoles 4 and 5 in THF with one equivalent of CH2Br2 in an ACE
pressure tube (Scheme 2) following standard procedures [25].
Palladium complexes 1 and 2 were prepared in good yields from
the corresponding bisimidazolium bromide salts 6 and 7 using
[Pd(OAc)2] as metal precursor in wet DMSO using an optimized
temperature program (Scheme 3).
ꢀ
non-coordinating PF6 ions.
These complexes were purified by filtration of their acetonitrile
solution over Celite, washed with water and subsequently recrys-
tallized from acetonitrile/diethyl ether, thereby removing the
remaining silver salts. This halogen exchange reaction follows a
slightly modified previously established procedure [60].
Unlike complexes 1–3 the dicationic species 8–10 can easily be
dissolved in polar aprotic solvents, like acetonitrile or DMSO. In
addition, the C„N vibrations of the coordinated acetonitrile, which
are isolobal [61] to the C„O vibrations normally used to measure
the TEP, can be unambiguously identified in their IR spectra.
The non-coordinating counterion PF6ꢀ can be used as part of the
auxiliary electrolyte in cyclovoltammetry. It also has a favorable
electrochemical window and was previously used for CV studies
of palladium NHC complexes [62].
Single crystals suitable for the determination of the solid state
structures of complexes 8–10 were obtained by slow diffusion of
2+
N
N
N
N
N
N
N
2.2 eq. AgPF6
2
CH2Br2
N
N
N
N
N
N
N
R
Pd
N N
Pd
Br Br
MeCN, ΔT
R
R
THF, 130 °C
2 Br
6 , 7
R
R
R
R
H3C
CH3
4 , 5
8 R = ortho-OCH3
2 PF6
4 R = ortho-OCH3 (6: 36%)
5
9
R = meta-OCH3
10 R = para-OCH3
7
R = meta-OCH3 ( : 71%)
Scheme 2. Synthesis of the new bisimidazolium salts 6 and 7.
Scheme 4. Synthesis of the dicationic complexes 8–10.

208
D. Munz et al. / Inorganica Chimica Acta 392 (2012) 204–210
3.4. Cyclovoltammetry
As cyclovoltammetric experiments by Amatore and Jutand have
shown for chelated phosphine complexes [64] and monodentate
NHC complexes [29], CV can be a valuable tool for mechanistic
investigations of catalytic cycles. Plenio used CV in order to study
the electronic properties of para phenyl substituted monodentate
NHC complexes [65–68]. He reasoned that through space p-inter-
actions might be important in certain cases and furthermore ob-
tained an excellent correlation of para Hammett substituent
constants and reduction potentials. We very recently became also
aware of two other electrochemical studies involving chelated bis-
NHC palladium(II) complexes [31,32].
This study focuses on the examination of position-dependent
(ortho, meta, para) electronic effects of methoxyphenyl substituted
bis-NHC ligands. By the direct preparation of the dicationic com-
plexes 8–10 and by using [n-Bu4N][PF6] as supporting electrolyte
we eliminated the problem of having different counterions which
might have different effects on the reaction.
A reduction peak was found for complexes 8–10 as shown in
Fig. 2. It corresponds to an irreversible two electron reduction of
palladium(II) to palladium(0) according to the results obtained
by Jutand et al. [29,70], van Huynh0s group [62] and Tubaro [31].
We were able to find a significant difference in the reduction
potentials for the three complexes (Table 2).
The reduction potentials clearly show that the para substituted
bis-NHC complex 10 (Table 2) needs a stronger negative potential
to be reduced than 8 and 9, which corresponds to a more electron-
rich palladium center. As the observed reduction potentials are
irreversible they clearly involve follow-up processes besides the
Fig. 1. ORTEP plot of
9 in the solid state, thermal ellipsoids drawn at 50%
probability level. Selected distances (Å) and angles (°): Pd1–C2 1.970(3); Pd1–C12
1.977(3); Pd1–N5 2.052(3); Pd1–N6 2.058(3); N6–C25 1.122(4); N5–C23 1.128(5);
C2–Pd1–N6 179.49(13); C12–Pd1–N5 174.43(13); N5–Pd1–N6 87.29 (11); Pd1–C2–
N1–C1 ꢀ0.2(4); Pd1–C12–N3–C1 ꢀ2.2(4); C2–N2–C5–C10 ꢀ53.8(5); C12–N4–C15–
C16 55.8(4).
diethyl ether into a saturated solution of the respective complexes
in acetonitrile. However, only the data set obtained from a single
crystal of 9 (Fig. 1) was of suitable quality for the deposition (see
Supp. material).
transfer of electrons to the metal and p-system.
In order to put into perspective the overall donor properties of
the chelated methoxyaryl biscarbene palladium complexes they
were compared to the corresponding halogenide substituted com-
plex [PdCl2(MeCN)2]. For [PdCl2(MeCN)2] a reduction potential of
ꢀ0.71 V was obtained.
3.3. Conformational behavior of complexes 1–3 and 8–10
The solid state structure reveals the characteristic boat confor-
mation of the 6-membered metallacycle (Pd–C–N–CH2–N–C),
which previously also had been reported for 3 [25]. 1H NMR spec-
troscopy indicates a change of the molecular conformation, which
can be observed as it is relatively slow on the NMR timescale. The
ambient temperature 1H NMR spectra of the dicationic complexes
8–10 show an AB spin system for the diastereotopic hydrogen
atoms of the bridging methylene group. NMR spectra at higher
temperatures reveal that the system converges into a singlet, cor-
responding to a ring flip of the central 6-membered metallacycle.
This ring flip has already been studied in detail by us and others
with related methylene bridged biscarbene complexes [25,63].
Quantum chemical calculations (BP86/6-31G(d)) were per-
formed on the three cationic complexes and the results were com-
pared with data from the solid state structures 8–10. Bond lengths
and angles calculated by gas phase DFT calculations are in good
agreement with the values derived from the diffraction data. Only
for the bonds of the weakly coordinated acetonitrile ligands, a
maximum deviation of 0.04 Å was found. The methoxy groups at
the phenyl ring point away from the central metallacycle in the so-
lid state structures (Fig. 1) as well as in the gas phase calculations.
The observation of two independent molecules in the unit cell of 8
and 9, differing slightly in the dihedral angle between the phenyl
3.5. NMR spectroscopy
A characteristic resonance is the carbene carbon signal in the
13C NMR spectra, which has successfully been used to characterize
10
0
-10
-20
j
-30
-1.89
-40
-50
-2.5
-2.0
-1.5
-1.0
E
/ V
rings and the heterocycle indicate
a certain conformational
Fig. 2. Cyclovoltammogram of 10 in acetonitrile in the presence of [n-Bu4N][PF6]
(c = 0.10 M) at 25 °C under argon at a scan rate of 100 mV sꢀ1. Potentials are
flexibility. The BP86 calculated minimum structures of complexes
8–10 exhibit an overall difference for that dihedral angle of up to
9° (complex 8: 51°, 9: 60°, 10: 56°). This small difference will not
influence the electronic communication between the rings
significantly.
referenced to the FeCp2/FeCp2 redox couple as internal standard (FeCp2 = Fe(g5
-
+
C5H5)2, E0 = 0.00 V)[69]. The 1st scan is shown in red, the dotted line shows the
control experiment, the supporting electrolyte solution only. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)

D. Munz et al. / Inorganica Chimica Acta 392 (2012) 204–210
209
expectation and together with results of the CV and 13C NMR stud-
ies we could show that a methoxy substituent in meta position will
have a negative effect on the donor properties, while the para
substituted complex 10 is the most electron-rich complex.
Table 2
Reduction potentials of complexes 8–10. Values are given in
Volt.
Complex
Ered (V)
8 (ortho)
9 (meta)
10 (para)
ꢀ1.69
ꢀ1.72
ꢀ1.89
4. Conclusion
Five new 3,30-bis(methoxyphenyl)-[(1,10-diimidazolin-2,20-diy-
lidene)methane] palladium(II) complexes have been prepared
and characterized. Due to their better solubility the ortho, meta
and para substituted isomers with hexafluorophosphate counteri-
ons were used to investigate electronic effects of substituents at
the aromatic ring. They extend our work on aryl substituted bis-
NHC ligands, which previously had been limited to substitution
in para position of the aryl ring. The effect of the position of meth-
oxy substitution on the electronic properties has been investigated
by cyclovoltammetry, IR, and NMR spectroscopy. The results indi-
cate that a variation of the position of the methoxy substituent has
a considerable electronic effect on the metal. The para substituted
complex was found by all characterization methods applied to
have the strongest donor properties. These results have also been
confirmed by DFT calculations.
the donor strength of different monodentate NHC ligands [71]. We
were able to obtain 13C NMR spectra of dicationic complexes 8–10
in DMSO at ambient temperature and could assign 13C shifts for the
carbene carbon atoms (Table 3).
The para methoxy substituted complex 10 has the lowest chem-
ical shift d. Although the differences in the 13C chemical shift values
are rather low, our results agree well with results recently pub-
lished by Tubaro et al. [31], who also obtained an upfield shift
for electron-rich methylene bridged palladium(II) bisimidazoliny-
lidene complexes compared to those with less electron density.
The observed ordering of the three complexes 8–10 predicts the
para substituted biscarbene complex 10 in accordance to the CV re-
sults to be the most electron-rich bis-NHC complex.
Acknowledgements
3.6. IR spectroscopy
Donor effects of ligands are often quantified by Tolman0s Elec-
tronic Parameter (TEP) [36], which is the IR absorption band corre-
sponding to the carbonyl C„O vibration of complexes of the type
Ni(CO)3L (L = monodentate 2-electron ligand) and has been trans-
ferred to CO complexes of the same coordination geometry [9].
As the RAC„N fragment can be described as an isolobal analogon
[61] to the C„O bond of carbon monoxide, we expected additional
information about the donor effects by the IR vibrations of the
C„N bonds of the coordinated acetonitrile molecules [72,73].
Therefore, the solid state ATR-FTIR spectra of 8–10 were measured
and compared to the calculated (BP86/6-31G(d)) harmonic IR spec-
tra (Table 4). The resonances corresponding to the IR vibration of
the CN groups can be clearly identified in the spectra, as there
are no other resonances of significant intensity close to the region
around 2300 cmꢀ1. We therefore could assign them to the sym-
metric and asymmetric bond stretch vibrations of the CN function-
ality as predicted by DFT (Table 4).
We thank the center of high-performance computing (ZIH) of
the TU Dresden for providing computing time. We cordially thank
Dr. M. Gruner, TU Dresden, for help with the NMR experiments.
The financial support of the Studienstiftung des Deutschen Volkes
(scholarship to D. M.) and of the Fonds der Chemischen Industrie
(scholarship to A.P.) is gratefully acknowledged.
Appendix A. Supplementary material
Supplementary crystallographic data and 3D-plots of 8 and 10,
IR spectra of 8–10, experimental details for the synthesis of imida-
zoles 4 and 5 as well as detailed computational results are avail-
able free of charge via the Internet. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2012.06.009.
The measured vibrational frequencies show only small differ-
ences, but as lower frequencies correspond to stronger donor prop-
erties, para substituted 10 shows the highest donor strength
among the methoxyphenyl substituted complexes, indicating an
electronic effect on complexes 8–10. These results confirm our
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