Synthesis and characterization of PdII-methyl complexes with N-heterocyclic carbene-amine ligands

Article abstract of DOI:10.1039/b906817a

A number of palladium(ii) complexes with a heteroditopic NHC-amine ligand and their precursor silver(i) carbene complexes have been efficiently prepared and their structural features have been investigated. The heteroditopic coordination of this ligand class was unequivocally shown by NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere rapid reductive elimination of the acyl-imidazolium salt was observed.

Full text of DOI:10.1039/b906817a

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N-Heterocyclic Carbenes
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Westfälische Wilhelms-Universität, Münster, Germany
Published in issue 35, 2009 of Dalton Transactions
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www.rsc.org/dalton | Dalton Transactions
Synthesis and characterization of PdII-methyl complexes with N-heterocyclic
carbene-amine ligands†
Stefan Warsink,a Sandra Y. de Boer,a Lianne M. Jongens,a Ching-Feng Fu,b Shiuh-Tzung Liu,b
Jwu-Ting Chen,b Martin Lutz,c Anthony L. Spekc and Cornelis J. Elsevier*a
Received 6th April 2009, Accepted 8th June 2009
First published as an Advance Article on the web 21st July 2009
DOI: 10.1039/b906817a
A number of palladium(II) complexes with a heteroditopic NHC-amine ligand and their precursor
silver(I) carbene complexes have been efficiently prepared and their structural features have been
investigated. The heteroditopic coordination of this ligand class was unequivocally shown by
NMR-spectroscopy and X-ray crystallographic analysis. The neutral and cationic
cis-methyl-palladium(NHC) complexes are not prone to reductive elimination, which is normally a
major degenerative pathway for this type of complex. In contrast, under carbon monoxide atmosphere
rapid reductive elimination of the acyl-imidazolium salt was observed.
Introduction
Research in the area of N-heterocyclic carbenes (NHC) has taken
a great flight in the past decade since the isolation of the first stable
carbene in 1991.1 This ligand class is nowadays able to compete
with the ubiquitous phosphane ligands. As more and more unique
characteristics were uncovered, the attention directed to these
versatile and relatively robust ligands is fully justified. Their use
as coordination compounds and as ligands in organometallic
chemistry and homogeneous catalysis is well documented, and
the understanding of their chemistry is increasing steadily.2
Combining strong and weak donors in one ligand may, by
virtue of the differentiation in relative trans-influences and effects,
lead to interesting reactivity of its transition metal complexes.
Such heteroditopic ligands may also lend stability to resting
states of catalytically active complexes, at the same time ensuring
temporary stabilization of active pre-catalysts. The NHC function
is usually strongly bound to the metal, whereas a second, weaker
donor moiety can dissociate in a reversible fashion, opening up a
coordination site for substrate binding and activation.
We have recently communicated the synthesis, characterization
and catalytic activity of a class of palladium(0) complexes with
such heteroditopic NHC-amine ligands.3 We now report the
corresponding complexes of carbene-amine ligands with divalent
palladium (Fig. 1).
Fig. 1 General structure of the reported complexes.
ligands lacking the tethered amine function. Furthermore, we are
interested in the possible degeneration of the NHC-complexes
by reductive elimination of hydrocarbyl-species where the alkyl
group would be cis or trans relative to the coordinated NHC
carbon. This degenerative pathway has been well documented by
Cavell and others4 and constitutes a potential disadvantage of the
use of NHCs in catalysis. To prevent reductive elimination from
occurring in such cases, biscarbene ligands or carbenes bearing
a strong secondary donor have been employed.2g In our system,
only a weakly coordinating secondary amine donor is present.
In this fashion, we expect to stabilize the complex while still
being able to retain activity in for example transfer and insertion
reactions, as well as hydrosilylation. The synthesis and reactivity
of the complexes and possible pathways for catalyst deactivation
are investigated.
It has been shown that the zero valent palladium compounds
[Pd(NHC-N)] are excellent catalysts for selective transfer semi-
hydrogenation of alkynes to cis-alkenes. A special feature of these
catalysts is that they contain an internal amine base, thereby
obviating the need to add a stoichiometric amount of base, as
was needed for simple Pd-NHC catalysts with simple carbene
Results and discussion
Indirect synthesis of neutral PdII-complexes
The synthetic route to the imidazolium ligand precursors and their
corresponding silver(I) complexes has been previously reported3
and a series of bis(carbene)silver complexes 1–3 was synthesized
in (near) quantitative yields for the ligands with ortho-substituents
on the N-aryl groups and yields of 76% for 2 and 3 (Scheme 1).
aUniversity of Amsterdam, van’t Hoff Institute for Molecular Sciences,
Nieuwe Achtergracht 166, 1018WV, Amsterdam, The Netherlands. E-mail:
c.j.elsevier@uva.nl; Fax: +31 20 525 6456; Tel: +31 20 525 5653
bDepartment of Chemistry, National Taiwan Chemistry, Taipei 106, Taiwan
1
The disappearance of the imidazolium proton at C-2 in the H
cFaculty of Science, Bijvoet Center for Biomolecular Research, Utrecht
University, Utrecht, The Netherlands
NMR is indicative for the formation of the Ag(NHC) complexes,
as is the shift of the signal for C-2 in the 13C NMR. For all
complexes, a downfield shift of about 40 ppm is observed, resulting
† CCDC reference numbers 726754 and 726755. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b906817a
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The transmetallation of 1a to divalent palladium precursors
was carried out smoothly and the products were obtained in high
yields and purity. Previously we reported the palladium dichloride
complex 43, in addition to which the palladium methylchloride
complex 5a has now been synthesized (Scheme 2).
Scheme 1 Synthesis of bis(NHC) silver complexes.
in a characteristic signal around 180 ppm. In mass spectrometry,
signals for mono- or bis(carbene)silver species are observed for
1–3.
As the exact structure of the silver-adducts cannot be com-
pletely established with only NMR and mass spectrometry, X-ray
analysis of a single crystal usually provides valuable additional
information.5
Hence, we grew X-ray quality crystals of 1c by slow diffusion of
pentane into a dichloromethane solution of the complex (Fig. 2).
Two cationic bis(NHC)Ag units are found in the unit cell, with
Scheme 2 Synthesis of neutral PdII complexes from 1a.
Complexes 4 and 5a have a characteristic signal in the 13C
NMR for the carbenic carbon at 175 and 171 ppm, respectively.
Proof of the coordination of the nitrogen-donor function has been
obtained from the resonances for the amino methyl-groups, for
2-
the complex anion Ag2Cl4 located on an inversion center. The
bis-carbene-silver(I) complex has a nearly linear geometry around
silver, with an C–Ag–C angle of 169.80(11)◦. The silver–carbene
distances are equal within experimental error and amount to
1
which a downfield shift is observed in the H NMR. The signals
˚
2.073(3) and 2.072(3) A, which are rather short Ag–C distances
for the hydrogens of the ethylene tether group clearly reveal an
for complexes with this geometry.6 The ligands are arranged in an
anti-parallel fashion, with the two carbene-rings disposed in an
almost co-planar arrangement. The mesityl-rings are twisted with
respect to the carbene-rings, the planes making angles of 77.6(4)◦
and 79.9(4)◦ for the two cases. There is no interaction of the
amine-donor with the silver-ion, neither is there any argentophilic
interaction between the cationic and anionic parts of the complex.
1
AA’BB’ second order pattern in the H NMR, confirming the
diastereotopicity of these protons. In the latter complex, the
heteroditopicity of the NHC-amine ligand was unequivocally
proved by the formation of only one of the possible geometrical
isomers, as indicated by 1H and 13C NMR. Upon coordination of
the carbene ligand, a large low-frequency shift is observed for the
1
PdMe signal. A shift of -0.15 ppm is observed in the H NMR
and at -12 ppm in the 13C NMR. Due to the difference in trans-
influence of the carbene and the amine-donor, we anticipated the
chloride to be in a position trans to the NHC.
Our expectations were confirmed by X-ray analysis of 5a,
crystals of which were grown by slow evaporation of a
dichloromethane solution (Fig. 3).
Fig. 2 Displacement ellipsoid plot of the cationic portion of 1c (50%
1
2
probability). Hydrogen atoms and [Ag2Cl4]2- anion are omitted for clarity.
Fig. 3 Displacement ellipsoid plot of 5a (50% probability).
Dalton Trans., 2009, 7080–7086 | 7081
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The chloride, being best acceptor ligand, is found trans to the
NHC moiety, just as the methyl and amine are in mutual trans
positions. The flexible linker of the bidentate ligand allows for a
nearly perfect square planar geometry of the complex, with angles
deviating no more than 3◦ from the ideal 90◦. The chelate ring itself
forms a twist-boat. The NHC is twisted by 37.5(2)◦ with respect
to the coordination plane and makes an angle of 67.2(3)◦ with the
The reaction times and use of solvents are significantly decreased,
making this process as a whole much more efficient. To explore the
versatility of this protocol, various imidazolium salts were used as
ligand precursors, resulting in complexes 5–10 (Scheme 3).
˚
mesityl-ring. The carbene-palladium distance is 1.991(2) A, which
is expected for a palladium(II)–NHC bond. The bond length of
˚
the methyl ligand to palladium is 2.046(2) A, which is quite short
compared to related structures.7 We attribute this to the weak
donor character of the amine in the trans position. The distance
˚
between the amine nitrogen and palladium is 2.2600(2) A. In
comparison with other divalent palladium complexes with amine
donors this is quite long8, reflecting the large electron density
imparted on the palladium by the NHC-ligand.
The thermal stability of the complex is remarkable because the
carbene and the methyl are in cis position relative to one another.
Together with the tilted geometry of the carbene with respect
to the coordination plane, this would be expected to facilitate
reductive elimination of the methylimidazolium salt.4 Apparently,
the amine donor provides already enough stabilization to prevent
this degenerative pathway from being accessible at room temper-
ature. To probe the sensitivity towards reductive elimination at
higher temperatures, an NMR study at higher temperatures was
performed. It appeared that 5a remained stable up to at least
Scheme 3 Direct synthesis of PdIIMeCl(NHC) complexes.
Results were generally very good, with yields ranging from
65 to 95%. In all cases, the formation of only the cis-methyl
NHC-complex was observed. Only in the case of the diisopropyl-
substituted amine, the product 5c was not obtained pure. In all
likelihood, the secondary donor introduces too much bulk near
the metal-center, leading to the formation of side products and
concomitant decomposition.
9
353 K in CD2Cl2 , which is quite striking. In earlier work, these
cis species were often found to decompose at room temperature or
below in dichloromethane.4 Until now, recourse had to be taken to
the use of biscarbenes or bidentate ligands bearing a strong donor
besides the carbene, in order to lend greater stability to these cis-
bis(hydrocarbyl) transition metal complexes7, whereas our system
features only a weakly coordinating secondary donor. The ease
with which the amine is, nevertheless, able to dissociate might very
well prove an important feature in catalytic applications where a
metal center with low coordination number is required.
Synthesis and reactivity of cationic PdII complexes
In addition to the neutral complexes, we were interested in the
synthesis and reactivity of cationic palladium-carbene species. Two
different cationic complexes were synthesized, by transmetallation
from 1a to [Pd(allyl)Cl]2 and by halide abstraction with silver(I)
triflate from 5a (Scheme 4).
II
ˆ
Direct synthesis of Pd (NHCN)MeCl complexes
Theoretically, the most time- and cost-efficient method to synthe-
size complexes bearing NHC-ligands is by direct complexation, i.e.
deprotonation of the imidazolium salt by a strong base followed
by in situ complexation to the desired metal. However, previous
research has shown that this route does not yield the products in
satisfactory yields for our ligands.3 The two-step method described
here gives the product in high yields and purities without the need
for stringent reaction conditions.
We have focused on a protocol that combines these good
results with the short reaction times generally associated with the
direct complexation. The in situ formation of the bis(NHC)silver
complex followed by addition of the metal precursor is well
documented, but we felt that this is in principle still a two-step
process.
We were satisfied to find that when imidazolium salt, silver(I)
oxide and palladium methyl chloride cyclooctadiene were sus-
pended in dry dichloromethane and stirred overnight, complex 5a
could be obtained in reasonable yield. Optimization revealed that
the use of activated molecular sieves brings about yields that are
comparable to those obtained with the optimized two-step process.
Scheme 4 Synthesis of cationic complexes.
Complex 11 was smoothly formed in a yield of 83%, and the
heteroditopicity was reflected in the spectroscopic behaviour of the
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1
allyl-ligand, for which five unique signals in the H NMR and 3
towards reductive elimination was observed for the neutral and
cationic methylpalladium complexes. This is remarkable because
the cis-geometry of the complex in combination with the tilted
conformation of the carbene normally favours a degenerative
pathway. This resilience is induced by the weak secondary donor.
However, the decomposition under influence of carbon monoxide
proved to be a facile process. X-Ray analysis of a representa-
tive (NHC)palladium(II) methylchloride complex was performed,
which unequivocally proved the heteroditopicity of the ligand.
Future research will exploit the striking stability of these
complexes, their reactivity and application in catalysis.
signals in the 13C NMR were observed. The signal for the carbene
carbon is found at 176 ppm in the 13C NMR. Additionally, a
broadening and splitting of the ortho-methyl groups on the mesityl
N-substituent is observed in NMR.
Another way to make reductive elimination a more realistic
option is to synthesize the cationic methylpalladium complex
by halide abstraction with silver. Starting from an acetonitrile
solution of 5a, immediate precipitation of silver(I) chloride was
observed on addition of silver(I) triflate. The solvent-complex 12
was obtained in a reasonable yield of 52% and high purity. For
these cationic complexes, decomposition was still not observed,
even after prolonged standing in dichloromethane solution. This
is again in contrast with earlier studies, which indicated that reduc-
tive elimination occurred more readily from cationic complexes.4
In contrast, when complex 5a in dichloromethane was subjected
to a carbon monoxide atmosphere, rapid deposition of palladium
black was observed. Analysis of the reaction mixture revealed the
complete disappearance of the palladium-methyl signal in the 1H
NMR and the appearance of a new signal at 2.8 ppm, suggesting
the formation of an acylimidazolium salt4e (Scheme 5). A possible
pathway for this degenerative process is exchange of the chloride
anion for CO, followed by migratory insertion and isomerisation
to the cationic cis-acyl carbene complex, from which reductive
elimination takes place. However, none of these intermediates were
observed in the reaction mixture. Another possible scenario is de-
coordination of the amine donor. In this case the methyl and
carbon monoxide ligands would end up in mutual trans positions.
In a complex with this geometry, formation of the acyl and con-
comitant reductive elimination is only possible after isomerization.
A third possibility is CO-insertion from a 5-coordinate species.10
To the best of our knowledge, palladium complexes bearing both
NHC and acyl ligands have not been isolated.4e
Experimental
All reactions involving air- or moisture-sensitive compounds were
carried out under an atmosphere of dry nitrogen. Solvents were
dried and distilled according to standard methods.11 palladium(II)
methyl chloride cyclooctadiene was prepared according to a litera-
ture procedure.12 NMR measurements were performed on a Varian
Mercury300 spectrometer (1H: 300.13 MHz, 13C 75.47 MHz), and
Bruker DRX300 spectrometer (1H: 300.13 MHz, 13C: 75.47 MHz).
13C NMR spectra were measured with 1H decoupling. Posi-
tive chemical shifts (d) are denoted for high-frequency shifts
relative to the external TMS reference. HRMS measurements
were performed on a JEOL JMS SX/SX102A four sector mass
spectrometer, coupled to a JEOL MS-MP9021D/UPD system
program. Samples were loaded in a matrix solution (3-nitrobenzyl
alcohol) on to a stainless steel probe and bombarded with xenon
atoms with an energy of 3 keV. During the high resolution
FAB-MS measurements a resolving power of 10 000 (10% valley
definition) was used.
General procedure for the synthesis of bis(NHC)silver(I) complexes
A procedure from the literature13 was modified as follows: silver(I)
oxide (1.0 equivalent) and the imidazolium salt (1.0 equivalent)
were suspended in dichloromethane to 0.2 M and stirred overnight
at room temperature. The reaction mixture was then filtered over
a pad of Celite, which was washed with 10 mL dichloromethane.
The filtrate was then concentrated.
Bis[(1-(2-N,N-dimethylamino)-ethylene-3-mesityl)imidazol-2-
ylidene] silver(I) dichloride (1a). The product (0.75 g, quantitative
yield) was obtained as a pale oil. H NMR (300 MHz, CD2Cl2)
Scheme 5 Decomposition of 5a under influence of CO with potential
reaction product.
1
d/ppm: 7.34 (d, 3J (HH) = 1.8 Hz, 1 H, CH), 6.99 (s, 2 H, aryl-H),
6.94 (d, 3J (HH) = 1.8 Hz, 1 H, CH), 4.22 (t, 3J (HH) = 6 Hz, 2 H,
N-CH2), 2.69 (t, 3J (HH) = 6 Hz, 2 H, Im-CH2), 2.34 (s, 3 H, p-aryl-
CH3), 2.25 (s, 6 H, o-aryl-CH3), 1.96 (s, 6 H, N(CH3)2). 13C NMR
(75 MHz, CD2Cl2) d/ppm: 181.8 (NCN), 140.0 (p-aryl-C), 136.4
(i-aryl-C), 135.2 (o-aryl-C), 129.9 (m-aryl-C), 123.0 (CH), 121.5
(CH), 59.3 (CH2), 50.5 (CH2), 45.5 (N(CH3)2), 22.3 (p-aryl-CH3),
18.7 (o-aryl-CH3). MS(FAB+): m/z = 623.2840 for C32H46N6Ag
[(NHC)2Ag]+.
The reactivity of NHC-bearing complexes does not necessarily
have implications for catalysis4, but the facile decomposition in
presence of CO does not bode well for application in for example
CO/alkene copolymerization.
Conclusions
We have synthesized a series of neutral and cationic palladium(II)
complexes with heteroditopic NHC-amine ligands, with various
substituents on the ligand aryl-groups and the amine, as well as the
other ligands on palladium. The complexes were prepared via two
routes: (a) initial formation of the bis(NHC)silver(I) complexes
followed by transmetallation and (b) direct complexation in a one-
pot procedure, which efficiently gives the desired product in yields
comparable to the two-step process. A high (thermal) stability
Bis[(1-(2-N,N-diethylamino)-ethylene-3-mesityl))imidazol-2-
ylidene] silver(I) dichloride (1b). The product (487 mg, 95%)
1
was obtained as greyish crystals. H NMR (300 MHz, CD2Cl2)
d/ppm: 7.32 (d, 3J (HH) = 1.5 Hz, 1 H, CH), 6.97 (s, 2 H,
3
aryl-CH), 6.92 (d, J (HH) = 1.5 Hz, 1 H, CH), 4.17 (bm, 2 H,
N-CH2), 2.77 (bm, 2 H, Im-CH2), 2.50 (q, 3J (HH) = 6.9 Hz, 4 H,
Dalton Trans., 2009, 7080–7086 | 7083
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2 CH2), 2.34 (s, 3 H, p-aryl-CH3), 1.95 (s, 6 H, o-aryl-CH3), 0.94
(t, 3J (HH) = 6.9 Hz, 6 H, 2 CH3). 13C NMR (75 MHz, CD2Cl2)
d/ppm: 181.3 (NCN), 139.7 (p-aryl-C), 136.2 (i-aryl-C), 135.3
(o-aryl-C), 129.6 (m-aryl-C), 122.9 (CH), 122.4 (CH), 54.6 (CH2),
51.2 (CH2), 47.8 (N(CH2)2), 21.3 (p-aryl-CH3), 17.8 (o-aryl-CH3),
12.5 (CH3). MS(FAB+): m/z = 392.1256 for C18H27N3Ag [M -
Cl]+.
[(1-(2-N ,N -dimethylamino)-ethylene-3-mesityl)imidazol-2-
ylidene] palladium(II) methyl chloride (5a). A procedure from the
literature13 was modified as follows: palladium(II) methyl chloride
cyclooctadiene (1.0 equivalent) and bis(imidazol-2-ylidene) silver
complex (0.5 equivalent) were suspended in dry dichloromethane
to 0.1 M and stirred at room temperature for 2 h under an
atmosphere of dry nitrogen. The reaction mixture was then
filtered over a pad of Celite, which was washed with 10 mL dry
dichloromethane. The solution was concentrated and the residue
was then washed three times with 10 mL dry diethyl ether. The
product (126 mg, 86%) was obtained as an off-white microcry-
stalline solid. H NMR (300 MHz, CD2Cl2) d/ppm: 7.09 (d, J
(HH) = 2 Hz, 1 H, CH), 6.96 (s, 2 H, aryl-CH), 6.79 (d, 3J (HH) =
2 Hz, 1 H, CH), 4.19 (m, 2 H, N-CH2), 2.63 (s, 6 H, N(CH3)2), 2.42
(m, 2 H, Im-CH2), 2.31 (s, 3 H, p-aryl-CH3), 2.18 (s, 6 H, o-aryl-
CH3), -0.16 (s, 3 H, PdCH3). 13C NMR (75 MHz, CD2Cl2) d/ppm:
170.9 (NCN), 139.1 (p-aryl-C), 136.6 (i-aryl-C), 134.9 (o-aryl-C),
129.4 (m-aryl-C), 123.1 (CH), 121.2 (CH), 63.3 (CH2), 49.4 (CH2,
N(CH3)2), 21.2 (p-aryl-CH3), 18.8 (o-aryl-CH3), -12.2 (Pd-CH3).
MS(FAB+): m/z = 378.1170 for C17H26N3Pd [M - Cl]+.
Bis[(1-(2-N,N-diisopropylamino)-ethylene-3-mesityl)imidazol-
2-ylidene] silver(I) dichloride (1c). The product (236 mg, quantita-
tive yield) was obtained as colourless crystals. 1H NMR (300 MHz,
3
1
3
CD2Cl2) d/ppm: 7.23 (d, J (HH) = 1.7 Hz, 1 H, CH), 6.99 (s,
2 H, aryl-H), 6.91 (d, 3J (HH) = 1.7 Hz, 1 H, CH), 4.14 (bs, 2 H,
3
i
N-CH2), 3.02 (septet, J (HH) = 6.6 Hz, 2 H, Pr-CH), 2.85 (bs,
2 H, Im-CH2), 2.34 (s, 6 H, p-aryl-CH3), 1.96 (bs, 6 H, o-aryl-CH3),
3
i
0.95 (d, J (HH) = 6.6 Hz, 12 H, Pr-CH3). 13C NMR (75 MHz,
CD2Cl2) d/ppm: 182.3 (NCN), 139.8 (p-aryl-C), 136.1 (i-aryl-C),
135.5 (o-aryl-C), 129.0 (m-aryl-C), 123.1 (CH), 122.1 (CH), 54.7
(CH2), 48.5 (CH2), 46.7 (N(CH)2), 21.5 (p-aryl-CH3), 21.0 (CH3),
17.8 (o-aryl-CH3). MS(FAB+): m/z = 420.1411 for C20H29N3Ag
[M - Cl]+.
General procedure for the one-pot synthesis of palladium(II) methyl
chloride NHC complexes
Bis[(1-(2-N-pyrrolidinyl)-ethylene-3-mesityl)imidazol-2-ylidene]
silver(I) dichloride (1d). The product (246 mg, 92%) was obtained
1
as a sticky yellow solid. H NMR (300 MHz, CD2Cl2) d/ppm:
Imidazolium salt, silver(I) oxide (1.0 equivalent) and palladium
methyl chloride cyclooctadiene (1.0 equivalent) were suspended
in dry dichloromethane and stirred with activated molecular
sieves overnight at room temperature under an atmosphere of
dry nitrogen. The reaction mixture was then filtered over a pad of
Celite, which was washed with 10 mL dry dichloromethane. The
filtrate was then concentrated and washed three times with 10 mL
dry diethyl ether.
3
7.35 (d, J (HH) = 1.8 Hz, 1 H, CH), 7.02 (s, 2 H, aryl-CH),
6.96 (d, 3J (HH) = 1.8 Hz, 1 H, CH), 4.30 (t, 3J (HH) = 6.0 Hz,
3
3
2 H, CH2), 2.93 (t, J (HH) = 6.0 Hz, 2 H CH2), 2.58 (m, J
(HH) = 2.4 Hz, 4 H, pyrrolidinyl), 2.37 (s, 3 H, p-aryl-CH3), 1.99
(s, 6 H, o-aryl-CH3), 1.77 (m, 3J (HH) = 1.8 Hz, 4 H, pyrrolidinyl).
13C NMR (75 MHz, CD2Cl2) d/ppm: 181.3 (NCN), 140.0
(p-aryl-C), 136.2 (o-aryl-C), 137.2 (i-aryl-C), 129.7 (m-aryl-C),
122.7 (CH), 122.1 (CH), 57.1 (pyrrolidinyl), 54.7 (CH2), 51.6
(CH2), 24.1 (pyrrolidinyl), 21.3 (p-aryl-CH3), 17.9 (o-aryl-CH3).
MS(FAB+): m/z = 673.3149 for C36H50N6Ag [(NHC)2Ag]+.
[(1-(2-N ,N -dimethylamino)-ethylene-3-mesityl)imidazol-2-
ylidene] palladium(II) methyl chloride (5a). The product (183 mg,
81%) was obtained as an off-white microcrystalline solid.
Bis[(1-(2-N,N-dimethylamino)-ethylene-3-(4-methoxyphenyl))
imidazol-2-ylidene] silver(I) dichloride (2). The product (284 mg,
[(1-(2-N ,N -diethylamino)-ethylene-3-mesityl)imidazol-2-
ylidene] palladium(II) methyl chloride (5b). The product (128 mg,
92%) was obtained as a yellow solid. 1H NMR (300 MHz, CD2Cl2)
d/ppm: 7.06 (d, 3J (HH) = 2.1 Hz, 1 H, CH), 6.98 (s, 2 H, aryl-
1
76%) was obtained as a brown/green oil. H NMR (300 MHz,
CD2Cl2) d/ppm: 7.49 (m, 2 H, aryl-H), 7.31 (d, 3J (HH) = 2.1 Hz,
1 H, CH), 7.24 (d, 3J (HH) = 2.1 Hz, 1 H, CH), 7.03 (m, 2 H, aryl-
3
H), 6.80 (d, J (HH) = 2.1 Hz, 1 H, CH), 4.25 (m, 2 H, CH2),
3
H), 4.26 (t, J (HH) = 6 Hz, 2 H, N-CH2), 3.87 (s, 3 H, OCH3),
3.40–2.80 (2 broad multiplets, 4 H, CH2), 2.66 (m, 2 H, CH2), 2.35
(s, 3 H, p-aryl-CH3), 2.17 (s, 6 H, o-aryl-CH3), 1.26 (t, 3J (HH) =
7.2 Hz, 6 H, CH3), -0.05 (s, 3 H, Pd-CH3). 13C NMR (75 MHz,
CD2Cl2) d/ppm: 169.7 (NCN), 139.2 (p-aryl-C), 136.6 (i-aryl-C),
135.1 (m-aryl-C), 129.3 (o-aryl-C), 123.3 (CH), 120.8 (CH), 53.4
(CH2), 52.8 (N(CH2)2), 50.7 (CH2), 21.2 (p-aryl-CH3), 18.8 (o-aryl-
CH3), 11.4 (CH3), -11.7 (Pd-CH3). MS(FAB+): m/z = 406.1486
for C19H30N3Pd [M - Cl]+.
2.72 (t, 3J (HH) = 6 Hz, 2 H, Im-CH2), 2.29 (s, 6 H, N-Me). 13
C
NMR (75 MHz, CD2Cl2) d/ppm: 179.8 (NCN), 160.4 (p-aryl-C),
133.7 (i-aryl-C), 126.0 (o-aryl-C), 122.4 (CH), 122.1 (CH), 115.1
(m-aryl-C), 60.4 (OCH3), 56.1 (CH2), 50.5 (CH2), 45.7 (N(CH3)2).
MS(FAB+): m/z = 352.0572 for C14H19N3OAg [M - Cl]+.
Bis[(1-(2-N,N-dimethylamino)-ethylene-3-(4-N,N-dimethyl-
phenyl))imidazol-2-ylidene] silver(I) dichloride (3). The product
(180 mg, 76%) was obtained as a brown solid. 1H NMR (300 MHz,
[(1-(2-N -pyrrolidinyl)-ethylene-3-mesityl)imidazol-2-ylidene]
palladium(II) methyl chloride (5d). The product (0.068 g, 66%)
3
CD2Cl2) d/ppm: 7.39 (d, J (HH) = 9 Hz, 2 H, aryl-H), 7.28
3
3
1
(d, J (HH) = 1.8 Hz, 1 H, CH), 7.21 (d, J (HH) = 1.8 Hz,
was obtained as a white powder. H NMR (300 MHz, CD2Cl2)
3
3
1 H, CH), 6.78 (d, J (HH) = 9 Hz, 2 H, aryl-H), 4.24 (t, J
(HH) = 9 Hz, 2 H, N-CH2), 3.01 (s, 6 H, aryl-N(CH3)2), 2.71 (d,
3J (HH) = 9 Hz, 2 H, Im-CH2), 2.29 (s, 6 H, N(CH3)2). 13C NMR
(75 MHz, CD2Cl2) d/ppm: 179.5 (NCN), 151.1 (p-aryl-C), 129.7
(i-aryl-C), 125.4 (o-aryl-C), 122.3 (CH), 122.1 (CH), 112.7 (m-aryl-
C), 60.5 (CH2), 50.5 (CH2), 45.9 (N(CH3)2), 40.8 (aryl-N(CH3)2).
MS(FAB+): m/z = 365.0893 for C15H22N4Ag [M - Cl]+.
d/ppm: 7.05 (d, 3J (HH) = 2.1 Hz, 1 H, CH), 7.00 (s, 2 H, aryl-
3
CH), 6.84 (d, J (HH) = 2.1 Hz, 1 H, CH), 4.22 (m, 2 H, CH2),
3.79 (m, 2 H, CH, pyrrolidinyl), 2.71 (m, 2 H, CH, pyrrolidinyl),
2.44 (m, 2 H, CH2), 2.35 (s, 3 H, p-aryl-CH3), 2.22 (s, 6 H, o-aryl-
CH3), 1.80 (m, 4 H, CH2, pyrrolidinyl), -0.13 (s, 3 H, Pd-CH3).
13C NMR (75 MHz, CD2Cl2) d/ppm: 170.0 (NCN), 139.3
(p-aryl-C), 137.3 (i-aryl-C), 134.8 (o-aryl-C), 129.6 (aryl-CH),
7084 | Dalton Trans., 2009, 7080–7086
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©

123.2 (CH), 120.9 (CH), 58.6 (pyrrolidinyl), 57.4 (CH2), 51.1
(CH2), 22.3, (pyrrolidinyl), 21.4 (p-aryl-CH3), 18.9 (o-aryl-CH3),
-12.5 (Pd-CH3). MS(FAB+): m/z = 404.1327 for C19H28N3Pd
[M - Cl]+.
2 H, aryl-H), 7.78 (d, 3J (HH) = 9.6 Hz, 2 H, aryl-H), 7.21 (d, 3J
(HH) = 2.1 Hz, 1 H, CH), 7.11 (d, 3J (HH) = 2.1 Hz, 1 H, CH),
4.24 (m, 2 H, CH2), 2.73 (s, 6 H, CH3), 5.24 (m, 2 H, CH2), -0.10 (s,
3 H, Pd-CH3). 13C NMR (75 MHz, CD2Cl2) d/ppm: 171.0 (NCN),
143.4 (p-aryl-C), 130.3 (CF3), 127.9 (i-aryl-C), 126.4, 126.1 (o-
and m-aryl-C), 122.3 (CH), 122.0 (CH), 62.8 (CH2), 49.7 (CH2),
49.4 (N(CH3)2), -7.3 (Pd-CH3). MS(FAB+): m/z = 404.0577 for
C15H19F3N3Pd [M - Cl]+.
(1-(2-N,N-dimethylamino)-ethylene-3-(4-methoxy) phenyl)-
imidazol-2-ylidene] palladium(II) methyl chloride (6). The
1
product (161 mg, 76%) was obtained as a pale yellow solid. H
NMR (300 MHz, CD2Cl2) d/ppm: 7.61 (d, 3J (HH) = 9 Hz, 2 H,
aryl-H), 7.13 (d, 3J (HH) = 2.1 Hz, 1 H, CH), 7.04 (d, 3J (HH) =
[(1-(2-N ,N -dimethylamino)-ethylene-3-mesityl)imidazol-2-
ylidene palladium(II) allyl] chloride (11). Palladium(II) allyl chlo-
ride dimer (68.5 mg, 0.19 mmol) and bis(imidazol-2-ylidene) silver
complex (157 mg, 1.0 equivalent) were suspended in 10 mL dry
dichloromethane and stirred at room temperature for 2 h under
an atmosphere of dry nitrogen. The reaction mixture was then
filtered over a pad of Celite, which was washed with 10 mL dry
dichloromethane. The solution was concentrated and the residue
was washed three times with 10 mL dry diethyl ether. The product
3
2.1 Hz, 1 H, CH), 7.00 (d, J (HH) = 9 Hz, 2 H, aryl-H), 4.20
(m, 2 H, CH2), 2.70 (s, 6 H, CH3), 2.48 (m, 2 H, CH2), -0.09
(s, 3 H, Pd-CH3). 13C NMR (75 MHz, CD2Cl2) d/ppm: 169.8
(NCN), 159.8 (p-aryl-C), 133.8 (i-aryl-C), 126.9 (o-aryl-C), 122.6
(CH), 121.5 (CH), 114.2 (m-aryl-C), 63.0 (CH2), 55.9 (OCH3),
49.6 (CH2), 49.4 (N(CH3)2), -8.2 (Pd-CH3). MS(FAB+): m/z =
366.0808 for C15H22N3OPd [M - Cl]+.
[(1-(2-N,N-dimethylamino)-ethylene-3-(4-N,N-dimethyl) phe-
nyl)imidazol-2-ylidene] palladium(II) methyl chloride (7). The
product (163 mg, 95%) was obtained as a brown solid. 1H NMR
(300 MHz, CD2Cl2) d/ppm: 7.48 (d, 3J (HH) = 9 Hz, 2 H, aryl-H),
7.07 (d, 3J (HH) = 1.8 Hz, 1 H, CH), 7.00 (d, 3J (HH) = 1.8 Hz,
1
(136 mg, 83%) was obtained as a yellow sticky solid. H NMR
(300 MHz, CD2Cl2) d/ppm: 7.57 (d, 3(HH) = 1.8 Hz, 1 H, CH),
3
7.00, (s, 2 H, aryl-H), 6.97 (d, (HH) = 1.8 Hz, 1 H, CH), 5.23
(m, 1 H, allyl), 4.64–4.34 (broad multiplet, 2 H, CH2), 4.05 (d,
3(HH) = 7.8 Hz, 1H, allyl), 3.19 (d, 3(HH) = 13.8 Hz, 1 H, allyl),
2.91 (m, 2 H, CH2), 2.78 (s, 6 H, CH3), 2.65 (broad multiplet, 1 H,
allyl), 2.35 (s, 3 H, p-aryl-CH3), 2.07 (broad singlet, 3 H, o-aryl-
CH3), 1.99 (broad singlet, 3 H, o-aryl-CH3), 1.78 (broad doublet,
3(HH) = 11.4 Hz, 1 H, allyl). 13C NMR (75 MHz, CD2Cl2) d/ppm:
176.1 (NCN), 139.9 (p-aryl-C), 136.9 (i-aryl-C), 135.7 (o-aryl-C),
135.3 (o-aryl-C), 129.4 (m-aryl-C). 129.3 (m-aryl-C), 123.3 (CH),
122.4 (CH), 118.6 (allyl), 74.4 (allyl), 62.8 (CH2), 51.7 (CH2),
48.6 (allyl), 48.5 (NMe2), 21.2 (p-aryl-CH3), 18.3 (o-aryl-CH3),
18.1 (o-aryl-CH3). MS(FAB+): m/z = 404.1332 for C19H28N3Pd
[M -Cl]+.
3
1 H, CH), 6.74 (d, J (HH) = 9 Hz, 2 H, aryl-H), 4.17 (m, 2 H,
N-CH2), 2.98 (s, 6 H, aryl-N(CH3)2), 2.67 (s, 6 H, N(CH3)2), 2.44
(m, 2 H, Im-CH2), -0.09 (s, 3 H, PdCH3). 13C NMR (75 MHz,
CD2Cl2) d/ppm: 169.2 (NCN), 150.6 (p-aryl-C), 130.0 (i-aryl-C),
126.4 (o-aryl-C), 122.7 (CH), 121.2 (CH), 111.9 (m-aryl-C), 63.1
(CH2), 49.5 (CH2), 48.8 (N(CH3)2), 40.7 (aryl-N(CH3)2), -8.5 (Pd-
CH3). MS(FAB+): m/z = 379.1125 for C16H25N4Pd [M - Cl]+.
[(1-(2-N ,N -dimethylamino)-ethylene-3-phenyl)imidazol-2-
ylidene] palladium(II) methyl chloride (8). The product (156 mg,
1
80%) was obtained as off-white crystals. H NMR (300 MHz,
CD2Cl2) d/ppm: 7.75–7.70 (m, 2 H, Ph), 7.54–7.50 (m, 3 H, Ph),
7.18 (d, 3J (HH) = 1.8 Hz, 1 H, CH), 7.09 (d, 3J (HH) = 1.8 Hz,
1 H, CH), 4.22 (m, 2 H, N-CH2), 2.71 (s, 6 H, N(CH3)2), 2.50
(m, 2 H, Im-CH2), -0.11 (s, 3 H, PdCH3). 13C NMR (75 MHz,
CD2Cl2) d/ppm: 170.1 (NCN), 140.8, 130.1, 128.7, 126.9 (Ph),
125.0 (CH), 122.3 (CH), 63.1 (CH2), 49.7 (CH2), 48.5 (N(CH3)2),
-8.1 (Pd-CH3). MS(FAB+): m/z = 336.0697 for C14H20N3Pd
[M - Cl]+.
[(1-(2-N ,N -dimethylamino)-ethylene-3-mesityl)imidazol-2-
ylidene palladium(II) methyl acetonitrile] trifluoromethane sulfonate
(12). To a solution of 5a (134 mg, 0.32 mmol) in 20 mL dry
acetonitrile was added silver(I) trifluoromethane sulfonate (1.1
equivalent) and the immediately formed white suspension was
stirred for 20 min at room temperature under an atmosphere of
dry nitrogen. The reaction mixture was then filtered over a pad
of Celite, which was washed with 10 mL dry dichloromethane.
The solution was then concentrated under reduced pressure. The
product (97 mg, 52%) was obtained as a grey sticky solid. 1H NMR
(300 MHz, CD2Cl2) d/ppm: 7.30 (d, 3(HH) = 2.1 Hz, 1 H, CH),
[(1-(2-N,N-dimethylamino)-ethylene-3-(4-acetyl)phenyl) imida-
zol-2-ylidene] palladium(II) methyl chloride (9). The product
(73 mg, 66%) was obtained as a pale yellow solid. 1H NMR
3
3
7.00, (s, 2 H, aryl-H), 6.91 (d, (HH) = 2.1 Hz, 1 H, CH), 4.30
(300 MHz, CD2Cl2) d/ppm: 8.09 (d, J (HH) = 8.7 Hz, 2 H,
3
3
(m, 2 H, CH2), 2.62 (s, 6 H, CH3), 2.56 (m, 2 H, CH2), 2.35 (s,
3 H, p-aryl-CH3), 2.32 (broad singlet, 3 H, MeCN), 2.16 (s, 6 H,
o-aryl-CH3), -0.13 (s, 3 H, Pd-CH3). 13C NMR (75 MHz, CD2Cl2)
d/ppm: 165.4 (NCN), 139.6 (p-aryl-C), 136.0 (i-aryl-C), 134.8 (or-
aryl-C), 129.5 (m-aryl-C), 124.1 (CH), 122.5 (CH), 120.0 (CN),
62.9 (CH2), 50.1 (NMe2), 49.4 (CH2), 21.2 (p-aryl-CH3), 18.7 (o-
aryl-CH3), 3.5 (Me-CN), -11.2 (Pd-Me). The signal for the triflate
was not observed. MS(FAB+): m/z = 419.1440 for C19H29N4Pd
[M - OTf]+.
aryl-H), 7.89 (d, J (HH) = 8.7 Hz, 2 H, aryl-H), 7.23 (d, J
(HH) = 2.1 Hz, 1 H, CH), 7.11 (d, 3J (HH) = 2.1 Hz, 1 H, CH),
4.23 (m, 2 H, CH2), 2.73 (s, 6 H, CH3), 2.62 (s, 3 H, acetyl-CH3),
2.52 (m, 2 H, CH2), -0.09 (s, 3 H, Pd-CH3). 13C NMR (75 MHz,
CD2Cl2) d/ppm: 197.2 (CO), 170.9 (NCN), 144.0 (p-aryl-C), 136.9
(i-aryl-C), 129.3 (o-aryl-C), 125.8 (m-aryl-C), 122.3 (CH), 122.0
(CH), 62.9 (CH2), 49.7 (CH2), 46.5 (N(CH3)2), 27.0 (CO-CH3),
-7.2 (Pd-CH3). MS(FAB+): m/z = 378.0809 for C16H22N3OPd
[M - Cl]+.
[(1-(2-N,N-dimethylamino)-ethylene-3-(4-trifluoromethyl) phe-
nyl)imidazol-2-ylidene] palladium(II) methyl chloride (10). The
X-Ray crystal structure determinations
1
product (107 mg, 69%) was obtained as an off-white solid. H
Reflections were measured on a Nonius KappaCCD diffractome-
ter with rotating anode (graphite monochromator, l = 0.71073 A)
3
˚
NMR (300 MHz, CD2Cl2) d/ppm: 7.94 (d, J (HH) = 9.6 Hz,
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7080–7086 | 7085
©

at a temperature of 150 K. Integration of the intensities
was performed with the programs EvalCCD14 (compound 1c)
or HKL200015 (compound 5a). The programs SADABS or
TWINABS16 were used for absorption correction and scaling. The
structures were solved with Direct Methods using the program
SHELXS-97.17 Refinement was performed with SHELXL-97
against F2 of all reflections. Non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen atoms were
introduced in calculated positions (compound 1c) or located in
difference Fourier maps (compound 5a) and refined with a riding
model. Geometry calculations and checking for higher symmetry
was performed with the PLATON program.18
program Taiwan-Netherlands, project no. 053.68.003, Han Peeters
for mass spectrometry and Jan Meine Ernsting for valuable
assistance with NMR experiments.
Notes and references
1 A. J. Arduengo III, R. H. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361.
2 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290; (b) F. E.
Hahn and M. C. Jahnke, Angew. Chem., Int. Ed., 2008, 47, 3122;
(c) E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Angew. Chem.,
Int. Ed., 2007, 46, 2768; (d) N. M. Scott and S. P. Nolan, Eur. J. Inorg.
Chem., 2005, 1815; (e) H. Jacobsen, A. Correa, A. Poater, C. Costabile
and L. Cavallo, Coord. Chem. Rev., 2009, 253, 687; (f) H. M. Lee,
C.-C. Lee and P.-Y. Cheng, Curr. Org. Chem., 2007, 11, 1491; (g) A. T.
Normand and K. J. Cavell, Eur. J. Inorg. Chem., 2008, 2781.
3 S. Warsink, P. Hauwert, A. M. Siegler, A. L. Spek and C. J. Elsevier,
Appl. Organomet. Chem., 2009, DOI: 10.1002/aoc.1501.
4 (a) K. J. Cavell and D. S. McGuiness, Coord. Chem. Rev., 2004, 248, 671;
(b) C. M. Crudden and D. P. Allen, Coord. Chem. Rev., 2004, 248, 2247;
(c) D. C. Graham, K. J. Cavell and B. F. Yates, Dalton Trans., 2006,
1768; (d) K. J. Cavell, Dalton Trans., 2008, 6676; (e) D. S. McGuinness
and K. J. Cavell, Organometallics, 2000, 19, 4918; (f) A. T. Normand,
A. Stasch, L. Ooi and K. J. Cavell, Organometallics, 2008, 27, 6507.
5 (a) J. C. Garrison and W. J. Youngs, Chem. Rev., 2005, 105, 3978;
(b) I. J. B. Lin and C. S. Vasam, Coord. Chem. Rev., 2007, 251,
642.
6 C. K. Lee, K. M. Lee and I. J. B. Lin, Organometallics, 2002, 21, 10;
W. Chen and F. Liu, J. Organomet. Chem., 2003, 673, 2363.
7 A. A. D. Tulloch, A. A. Danopoulos, R. P. Tooze, S. M. Cafferkey,
S. Kleinhenz and M. B. Hursthouse, Chem. Commun., 2000, 1247;
A. A. D. Tulloch, S. Winston, A. A. Danopoulos, G. Eastham and M. B.
Hursthouse, Dalton Trans., 2003, 699; N. Tsoureas, A. A. Danopoulos,
A. A. D. Tulloch and M. E. Light, Organometallics, 2003, 22, 4750.
8 W. de Graaf, J. Boersma, W. J. J. Smeets, A. L. Spek and G. van Koten,
Organometallics, 1989, 8, 2907.
Crystallographic data for 1c. [C40H62AgN6][Ag2Cl4]1 , FW =
2
-1
3
¯
913.60 g mol , yellow block, 0.48 ¥ 0.45 ¥ 0.15 mm , triclinic, P1
˚
(no. 2), a = 11.2826(3), b = 13.6358(3), c = 15.4197(4) A, a =
◦
3
˚
83.075(1), b = 77.757(1), g = 69.874(1) , V = 2173.83(10) A ,
Z = 2, Dx = 1.396 g cm-3, m = 1.057 mm-1. 33 419 reflections were
-1
˚
measured up to a resolution of (sinq/l)max = 0.61 A . The crystal
consisted of two fragments with arbitrary orientation with respect
to each other (2.6◦ approximately about the vector uvw = [1.93,
-5.00, -0.13]). The integration was therefore performed on the
basis of two orientation matrices. Absorption correction range
0.70–0.85. 8097 reflections were unique (Rint = 0.025), of which
7103 were observed [I > 2s(I)]. 466 parameters were refined
with no restraints on the basis of the non-overlapping reflections
of the first crystal fragment and the overlapping reflections of
both fragments using a HKLF5 file.19 R1/wR2 [I > 2s(I)]:
0.0323/0.0673. R1/wR2 [all reflections]: 0.0422/0.0738. S = 1.109.
Batch scale factor 0.152(2). Residual electron density between
-3
˚
9 S. Gaemers, H. Luyten, J. M. Ernsting and C. J. Elsevier, Magn. Reson.
Chem., 1999, 37, 25.
-1.52 and 1.26 e A .
Crystallographic data for 5a. C17H26ClN3Pd, FW = 414.26,
10 R. E. Ru¨lke, J. G. P. Delis, A. M. Groot, C. J. Elsevier, P. W. N. M. van
Leeuwen, K. Vrieze, K. Goubitz and H. Schenk, J. Organomet. Chem.,
1996, 508, 109.
colourless block, 0.21 ¥ 0.21 ¥ 0.12 mm3, monoclinic, P21/c (no.
˚
14), a = 13.1723(1), b = 11.1113(2), c = 14.7333(2) A, b =
11 D. D. Perrin, L. F. Armagero, Purification of Laboratory Chemicals,
◦
3
-3
˚
121.8193(6) , V = 1832.31(5) A , Z = 4, Dx = 1.502 g cm , m =
Pergamon Press, Oxford, 1998.
1.159 mm-1. 18 520 reflections were measured up to a resolution
12 K. J. Cavell, D. J. Stufkens and K. Vrieze, Inorg. Chim. Acta, 1981, 47,
67.
13 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972.
14 A. J. M. Duisenberg, L. M. J. Kroon-Batenburg and A. M. M. Schreurs,
J. Appl. Crystallogr., 2003, 36, 220.
15 Z. Otwinowski, W. Minor, Methods in Enzymology, ed. C. W. Carter,
Jr. and R. M. Sweet, Academic Press, London, 1997, vol. 276,
p. 307.
-1
˚
of (sinq/l)max = 0.65 A . Absorption correction range 0.51–
0.87. 4158 reflections were unique (Rint = 0.043), of which 3450
were observed [I > 2s(I)]. 205 parameters were refined with
no restraints. R1/wR2 [I > 2s(I)]: 0.0280/0.0643. R1/wR2 [all
reflections]: 0.0398/0.0687. S = 1.070. Residual electron density
-3
˚
between -0.75 and 0.74 e A .
16 G. M. Sheldrick, SADABS and TWINABS: Area-Detector Absorption
Correction, 1999, Universita¨t Go¨ttingen, Germany.
17 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008,
64, 112.
Acknowledgements
18 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
We thank the Netherlands Organization for Scientific Research
(NWO) for financial support within the bi-national research
19 R. Herbst–Irmer and G. M. Sheldrick, Acta Crystallogr., Sect. B:
Struct. Sci., 1998, 54, 443.
7086 | Dalton Trans., 2009, 7080–7086
This journal is
The Royal Society of Chemistry 2009
©