Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

Article abstract of DOI:10.1039/c3dt32835j

A transmetallation route, using silver(i) precursors, to several zero- and di-valent palladium complexes with chelating bis(N-heterocyclic carbene) ligands bearing various N-substituents has been established. The resulting complexes have been characterized by NMR and mass spectroscopy. In addition, the structure of a representative compound, [Pd⁰(bis-(Mes)NHC)(η²- ma)] (3a), was confirmed by X-ray crystal structure determination. In contrast to the transfer semihydrogenation, in which only low activity was observed, complex 3a showed activity (TOF = 49 mol_sub mol_cat ^-1 h^-1) and selectivity comparable to its monodentate counterparts in the semihydrogenation of 1-phenyl-1-propyne with molecular hydrogen.

Full text of DOI:10.1039/c3dt32835j

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Synthesis of Palladium(0) and ꢀ(II) Complexes with Chelating Bis(Nꢀ
Heterocyclic Carbene) Ligands and their Application in
Semihydrogenation
Soraya N. Sluijtera, Stefan Warsinka, Martin Lutzb and Cornelis J. Elseviera*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A transmetallation route, using silver(I) precursors, to several zeroꢀ and diꢀvalent palladium complexes
with chelating bis(Nꢀheterocyclic carbene) ligands bearing various Nꢀsubstituents has been established.
The resulting complexes have been characterized by NMR and mass spectroscopy. In addition, the
10 structure of a representative compound, [Pd0(bisꢀ(Mes)NHC)(η2ꢀma)] (3a), was confirmed by Xꢀray
crystal structure determination. In contrast to the transfer semihydrogenation, in which only low activity
was observed, complex 3a showed activity (TOF = 49 molsubmolcatꢀ1hꢀ1) and selectivity comparable to its
monodentate counterparts in the semihydrogenation of 1ꢀphenylꢀ1ꢀpropyne with molecular hydrogen.
15 Introduction
Following the success of their monodentate analogues, chelating
bis(Nꢀheterocyclic carbenes) (bisꢀNHCs) have become popular
ligands in transition metal catalysis.1,2 Because of the chelate
effect these bisꢀNHCs lead to metal complexes with higher
20 stability compared to their monodentate (bisꢀ)NHC counterparts
(Figure ).1 In addition to the advantages regarding stability, bisꢀ
NHCs are good candidates for fineꢀtuning of catalytic properties
by altering the wingtips, backbone and linker,3 the first of which
is explored in this contribution. The influence of the linker length
25 is briefly studied as well.
Scheme 1 General Synthesis of bisimidazolium salts. Mes
trimethylphenyl.
= 2,4,6ꢀ
BisꢀNHCs have been coordinated to many transition metals:
ruthenium,4 rhodium,3,5 iridium,6 palladium,7 platinum,8,9 and
more recently also first row metals such as nickel10–12 and
iron.13,14 We are interested in the properties and reactivity of lowꢀ
30 valent late transition metal complexes, notably electron rich zeroꢀ
valent palladium NHC complexes, as these often play a vital role
in the catalytic cycles of many reactions.15–18 Although a lot of
research has been devoted to bisꢀcarbene PdII(halide)2 complexes,
usually obtained by treating a bisimidazolium salt with Pd(OAc)2
35 at elevated temperatures,7,19–23 we are not aware of any previous
reports on chelate [Pd0(bisꢀNHC)L] or [PdII(bisꢀNHC)(η3ꢀ
C3H5)]Cl complexes. Moreover, reports on zeroꢀvalent d10 metal
complexes bearing a chelating bisꢀNHC ligand have not appeared
in literature at all.
40
Results
Synthesis of bisimidazolium salts
The synthesis of bisimidazolium salts, 1, has been documented;
reacting a mono substituted imidazole with 0.5 equivalent of
45 dihaloalkane in several solvents at elevated temperatures gives
the hygroscopic diimidazolium salts in decent yields (Scheme
1).24,25 However, yields are dramatically improved for the
methylene linked bisimidazolium salts (1a, cꢀe) by heating the
corresponding imidazole in an excess of dibromomethane,
Figure 1 General structure monodentate bisꢀNHC and bidentate bisꢀNHC
complex.
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Scheme 2 Synthesis of [Pd0(bisꢀNHC)(η2ꢀma)] 3 and [PdII(bisꢀNHC)(η3ꢀC3H5)]Cl 4 complexes via the silver complexes.
following a procedure by Lee et al.23
Table 1 Chemical shifts and multiplicity in NMR spectra of Pd0 and
PdII complexes, 3 & 4.
Silver and palladium complexes
Complex CCarbene BackboneꢀH
LinkerꢀH
(ppm)
187
191
ꢀa
(ppm)
(ppm, multiplicity)
5
Direct complexation by reacting the bisimidazolium salts with
KOtBu in the presence of Pd0 and PdII precursors
([Pd(tBuDAB)(η2ꢀma)] (tBuDAB = 1,4ꢀDiꢀtertꢀbutylꢀ1,4ꢀdiazaꢀ
1,3ꢀbutadiene, ma = maleic anhydride), [Pd(dvtms)]2 (dvtms =
divinyltetramethylsiloxane) or [Pd(η3ꢀC3H5)ꢁꢀCl]2), failed to give
3a
3b
3c
3d
3e
4a
4b
4c
4d
4e
7.38, 6.69
7.13, 6.93
7.68, 7.31
7.11, 6.93
7.10, 6.96
8.35, 6.99
7.55, 6.82
8.11, 6.96
8.06, 6.93
8.01, 7.04
6.54, 5.97 (d)
5.29, 4.34 (dd)
6.52, 5.71 (d)
5.89 (dd)
186
184
177
181
175
176
175
10 the palladium complexes in satisfactory yields. Instead, a
carbene transfer route via Ag(I) complexes was applied, which
was first adopted for the synthesis of zeroꢀvalent palladium
complexes in 200926 and showed excellent yields for chelate bisꢀ
NHC PdII complexes.12,27
5.81 (dd)
7.46, 6.56 (d)
5.29ꢀ5.07 (m)b, 3.04 (dd)
7.05 , 6.19 (d)
7.52, 6.46 (bs)
6.82, 6.12 (bs)
15
The silver(I) complexes, 2, were obtained by reacting the
corresponding bisimidazolium dibromide salts (1aꢀe) with Ag2O
at room temperature in dichloromethane (R = Mes) or methanol
35
a) Not detectable, even with prolonged relaxation times.
b) Due to overlap with the signal of an allylic hydrogen the
expected doublet of doublets could not be observed
(R = Me, Benz, nBu) (Scheme 2Error! Reference source not
29
found.) according to literature procedures.28,
Complete
20 conversion can be monitored by the disappearance of the
1
imidazoliumꢀH peak around 9ꢀ10 ppm in the H NMR spectrum
maleic anhydride complexes (42ꢀ99% yield) (Scheme 2Error!
13
and the
C
ꢀAg(I) resonance around 180 ppm in 13C NMR.
40 Reference source not found.). The lower yields for the
complexes with alkyl Nꢀsubstituents (3c and 3e) can be explained
by the instability of their Ag(I) precursors, demonstrated by
Quezada et al,30 as well as the low stability of the complexes
themselves (vide infra).
The resulting complexes, 3, were characterized by NMR
spectroscopy and mass spectroscopy (FAB+). It is reported that
the mode of coordination (chelating or bridging) of bisꢀcarbenes
is dictated by a combination of linker, Nꢀsubstituents, counter ion
and reaction conditions.22,31 It has even been doubted whether
NHC
The silver complex from the mesityl bisimidazolium salt forms
oligoꢀ and polymeric structures as was shown by Wanniarachi et
25 al.,28 while other substituents lead to dimeric complexes.30 The
reaction mixtures containing the silver(I) bisꢀcarbene complexes
were filtered over celite, concentrated and used in the subsequent
reaction without further purification.
Transmetallation of the Ag(I)NHC precursors, by reacting 2aꢀe
30 with one equivalent of [Pd(tBuDAB)(η2ꢀma)] relative to the bisꢀ
carbene ligand in dichloromethane at room temperature, provided
a convenient route to the corresponding zeroꢀvalent palladium
45
50 chelated methylene linked bisꢀNHC ligands would be able to
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support a zeroꢀvalent palladium center.32 However, we only 20 analogue (193 ppm).35
It appeared that the stability of these complexes depends on the
Nꢀsubstituents. Complex 3c and 3e decomposed in solution and
also for 3d formation of palladium black was observed after a
couple of hours in dichloromethane. This instability has been
25 observed for zeroꢀvalent monoꢀcarbene palladium complexes
bearing alkyl substituents as well,36 and can be attributed to the
absence of steric protection of the electronꢀrich metal center. The
decomposition of 3c in solution hampered the detection of the
carbene signal in 13C NMR spectroscopy. The complexes with the
30 bulkier mesityl substituents, that have been used previously to
ensure stability of palladium complexes bearing these
ligands,32,36,37 were stable in solution and resistant against air and
moisture at room temperature for at least a week. In contrast to
findings of Riederer et al.,3 we did not observe a relation between
35 stability and linker length.
a)
[PdII(bisꢀNHC)(η3ꢀC3H5)]Cl complexes, 4, have been
synthesized via the same route as their zeroꢀvalent counterparts
(Scheme 2). Transmetallation from the corresponding
silver(I)complexes proceeded in reasonable to high yields. The
40 resulting cationic complexes were all stabile in solution and air.
The reduced electron density on the metal of these stable
palladium(II) complexes is reflected in their NMR spectra
through a downfield shift of signals for the backbone hydrogens
and an upfield shift for the signal of the carbeneꢀcarbon compared
45 to the zeroꢀvalent counterparts (Tabel 1). In the 13C NMR spectra
two signals are present for the allyl group, indicating a symmetric
structure for the complexes and a nonꢀcoordinating chloride
anion. Again, the hydrogens of the linker give rise to two
b)
1
doublets in H NMR spectroscopy for 4a and 4c, and doublet of
Figure 2 a) Molecular structure of complex 3a in the crystal, drawn at the
50% probability level. Hydrogen atoms and dichloromethane solvent
molecule are omitted for clarity. Selected bond distances (Å) and (dihedral)
angles (°): Pd1ꢀC12: 2.046(2); Pd1ꢀC16: 2.045(2); C26ꢀC27: 1.445(3); C16ꢀ
Pd1ꢀC12: 87.50(9); C12ꢀPd1ꢀC26: 115.26(9); C27ꢀPd1ꢀC16: 116.64(10);
C26ꢀPd1ꢀC27: 40.18(9); Pd1ꢀC12ꢀC16ꢀC26ꢀC27^N1ꢀN2ꢀC10ꢀC11ꢀ12:
27.54(12); Pd1ꢀC12ꢀC16ꢀC26ꢀC27^N3ꢀN4ꢀC14ꢀC15ꢀ16: 23.64(13); Pd1ꢀ
C12ꢀC16ꢀC26ꢀC27^C26ꢀC27ꢀC28ꢀC29ꢀO2: 77.59(12). b) Conformation of
the sixꢀmembered metallacycle in 3a and orientation of η2ꢀmaleic anhydride
with respect to Pd plane. Mesityl substituents and hydrogens are omitted for
clarity.
50 doublets for 4b, indicating the inequivalence of these hydrogens
and the nonꢀfluxional behavior of the complexes at room
temperature. Exceptions are complexes 4d and 4e, that show two
broad singlets around 7.52, 6.46, 6.82 and 6.12 ppm respectively,
indicating fluxionality that could be caused by a lower barrier to
55 allyl rotation. Variable temperature 300MHz 1H NMR
experiments showed that, according to expectation, these
resonances become doublets upon cooling to ꢀ20°C for 4e in
dichloromethane, whereas the allyl peaks broaden.
observed formation of chelate species for all wingtip substituents
and for both linker lengths, as was concluded from the NMRꢀ
spectra. In the H NMR spectra, the signals of the methylene
60 Solid state structures of 3a
1
Xꢀray quality crystals of 3a were obtained by slow vapor
diffusion of pentane to a concentrated dichloromethane solution
of the complex. The Xꢀray crystal structure (Figure 2a) confirms
that the palladium center is coordinated to the bisꢀcarbene in a
65 chelating fashion. The complex adopts a planar geometry around
the transition metal center, while the sixꢀmembered metallacycle,
formed by the chelating bisꢀcarbene and palladium center, shows
a boat conformation (Figure 2b). The NHCs are bent out of the
metal coordination plane with dihedral angles of up to 28o. The
70 bisꢀcarbene bite angle of the complex is 87.50(9)o, a small
deviation from the ideal angle dictated by the methylene bridge.39
The palladium(0)ꢀcarbene bond distances of 3a are shorter
than those of the recently reported Yꢀshaped [Pd0(bisꢀNHC)(ma)]
complex containing two dissymmetric monodentate NHC ligands
75
5
linker hydrogens are split in two doublets with a geminal
coupling of approximately 13 Hz (Table 1). This observation can
be attributed to the complexes’ rigid boat structure, having a
plane of symmetry bisecting the bisꢀcarbene ligand and maleic
anhydride in all cases. For 3b (n=2) the same rigidity was
10 observed in the 1H NMR spectrum, having two doublets of
doublets assigned to the ethyleneꢀlinker hydrogens. The 1H NMR
spectrum of 3e exhibits diastereotopic hydrogens for the nBu
groups displaying their nonꢀequivalence as was observed before
for similar complexes.5,31 The orthoꢀmethyl groups and metaꢀ
15 hydrogens on the mesityl (3a) group both give rise to two singlets
1
13
in H NMR as well. The
C
ꢀPd0 resonances all fall in the
NHC
expected range (187ꢀ191 ppm) corresponding to shifts of
monodentate bisꢀcarbene Pd0(η2ꢀma) equivalents (185ꢀ189
ppm),33, 34 and slightly upfield compared to their monoꢀcarbene
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100
90
80
70
60
50
40
30
20
10
0
%
conversion
alkyne
alkane
Scheme 3 Semihydrogenation of 1ꢀphenylꢀ1ꢀpropyne to products. Zꢀ
alkene is preferred.
Zꢀalkene
Table 2 Catalytic results semihydrogenation of 1ꢀphenylꢀ1ꢀpropyne.
Eꢀalkene
entrCat. Hydrogen T (oC) Tim Convers
Zꢀ/Eꢀalkene/ alkane
y
source
e (h) ion
(%)
0
200
400
600
800
two hours (TOF = 49 molsub/molcat/h). Moreover, 3a shToiwmeed(mhinig.)h
1
2
3a
3a
HCO2H/NEt3
H2
70 29
60
18
98/2/0
89/4/6
3
>99
Figure 3 Selectivity of catalyst 3a in the semihydrogenation of 1ꢀphenylꢀ
1ꢀpropyne over time.
a.) Reaction conditions: 2.2 mmol substrate, <1 mol% catalyst; for entry
1ꢀ2: 5 equiv. HCO2H/NEt3; for entry 3ꢀ4: p(H2) = 1 bar. b.) Product
distribution as determined by GC at 18% and 99% conversion
respectively.
5
selectivity towards the Zꢀalkene up to 80% conversion. When
most of the alkyne was consumed, overꢀreduction to the alkane
50 was observed (Figure 3). The catalyst was still active when
adding new substrate after almost full conversion of the alkyne.
selectivity and reaction rate.41,42,48 Catalytic tests with 4a in both
transfer and direct semihydrogenation led to disappointing
results, giving no conversion at all. The reason for this lack of
55 catalytic activity may be that the allyl group is not easily
hydrogenated. Currently, we are further exploring the possible
applications of these complexes.
(2.061(2)ꢀ2.067(3) Å)33, but are in agreement with the values of
chelate heteroditopic zeroꢀvalent palladium complexes
(2.0315(14)ꢀ2.045(2) Å).26,40–42 In addition, Lee et al. reported
10 unequal palladiumꢀcarbene lengths within their zeroꢀvalent
monodentate bisꢀcarbene palladium complexes in the same range
(2.043(4)ꢀ2.070(3) Å).43 The C=C double bond of the maleic
anhydride, which is η2 coordinated to the metal center and almost
perpendicular to the coordination plane (Figure 2Error!
15 Reference source not found.), is elongated compared to the free
alkene due to πꢀback bonding from the palladium(0) center.
Suitable crystals of 4a were obtained as well, by slow vapor
diffusion of pentane to a concentrated dichloromethane solution
of the complex, but the structure could not be solved
20 satisfactorily. A large number of solvent molecules were present
in the crystal structure, which led to a high degree of disorder.
The results did however show unequivocally that the carbene was
coordinated in a bidentate fashion and that the chloride anion was
not coordinated to the metal center.
Conclusion
We have described the synthesis and characterization of the first
60 chelate bisꢀNHC palladium(0) and palladium(II) allyl complexes
with various substituents on the wingtip position. Also the length
of the linker moiety was varied. Except for the Pd0(bisꢀNHC)
complexes bearing alkyl substituents, the formed bisꢀcarbene
complexes were stable to air and in solution.
65
In contrast to their monoꢀcarbene analogues these complexes,
that lack an additional coordination site, proved to be inactive in
the transfer semihydrogenation of 1ꢀphenylꢀ1ꢀpropyne. On the
other hand the zeroꢀvalent palladium bisꢀcarbene complex 3a
does catalyze the reaction when molecular hydrogen (1 bar) is
25
Catalytic semihydrogenation of 1ꢀphenylꢀ1ꢀpropyne
Palladium NHC complexes have been successfully applied in
many reactions,44–46 including the (transfer) semihydrogenation of
alkynes (Scheme Error! Reference source not found.3).15,30,34,41
30 We applied complex 3a in this reaction, using 1ꢀphenylꢀ1ꢀ
propyne as substrate and formic acid
70 used and comparable activity and initial selectivity is observed as
in systems using monoꢀcarbene palladium complexes. The PdII
allyl equivalent showed no conversion for the semihydrogenation
under the same conditions, which can be due to the reluctance of
the allyl coligand to undergo dissociative processes or reduction
75 to the active palladium(0) species. Possibly, since they constitute
in principle (re)active organometallic preꢀcatalysts, the current
and similar new Pd(bisꢀNHC)(L) complexes will be amenable to
carbonꢀcarbon and other carbonꢀelement coupling reactions
Bisꢀcarbene complex 3a showed good activity and selectivity
in the semihydrogenation of 1ꢀphenylꢀ1ꢀpropyne. However, the
results were similar to previously published systems in terms of
35 as a hydrogen donor at 70oC in acetonitrile. Unfortunately, this
complex showed poor activity of only 18% conversion for this
system (Table 2, entry 1). Considering the proposed mechanism
for the monoꢀNHC analogue in which both formic acid and the
alkyne coordinate to the Pd0 center,15 this may be explained by
40 the presence of only one available vacant site in this system,
whereas two are needed if the same mechanism would hold here.
When hydrogen gas was used as the source of hydrogen, the
complex proved to be more active (Table 2, entry 2). After an
induction period of approximately four hours, presumably needed
45 for sufficient alkene dissociation or hydrogenation in order to
create a vacant site, the reaction proceeded to completion within
Experimental
80 All reactions were carried out using standard Schlenk techniques
under an atmosphere of dry nitrogen. Solvents were dried and
distilled according to standard methods.49 All reagents were
purchased from commercial suppliers and used without further
purification. The following starting materials were synthesized
85 according to literature procedures: mesitylimidazole,50
butylimidazole,51 the bisimidazolium dibromides23 and
4
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[Pd(tBuDAB)(η2ꢀma)]52. The silver(I) complexes were also
CH2), 3.82 (2H, s, ma) 3.36 (6H, s, CH3). 13C NMR (75 MHz,
prepared following reported procedures; in MeOH for R = Me, 60 CD2Cl2): 171.9 (CO), 124.5 (CH) , 123.7 (CH), 64.4 (CH2), 39.5
Benz, nBu29 or dichloromethane for R = Mes.28 The NMR spectra
were recorded on Varian Mercury 300 MHz, Bruker DRX 300
and Bruker AMX 400 MHz spectrometers. Data for NMR are
(alkene), 38.0 (CH3). MS(FAB+) for C9H12N4Pd: m/z calculared
282.0100 [Mꢀma]+, obeserved 282.0121.
5
reported as follows: chemical shift in parts per million (δ, ppm)
Methylꢀ1,1ꢀdi(benzylimidazolꢀ2ꢀylidene) palladium (0) maleic
1
1
relative to TMS, determined by reference to residual H and 13C 65 anhydride, 3d: Brown solid, 83 mg, 0.16 mmol, 98%. H NMR
solvent resonances, where applicable multiplicity (s = singlet, d =
doublet, t = triplet, q = quartet and m = multiplet), number of
(CD2Cl2, 300 MHz): 7.41ꢀ7.26 (10H, m, arylꢀH), 7.11 (2H, d,
3
3JHH = 1.8 Hz, CH), 6.93 (2H, d, JHH = 1.8 Hz, CH), 5.89 (2H,
2
2
10 hydrogens and coupling constant (Hz). Mass spectra were
recorded on JEOL JMS SX/SX102A fourꢀsector mass
dd, JHH = 17 Hz, CH2), 5.41 (2H, d, JHH = 2.7 Hz, CH2), 3.69
a
(2H, s, ma). 13C NMR (CD2Cl2, 75 MHz): 186.5 (NCN), 174.7
spectrometer and GC analyses were performed with an 70 (CO), 138.5 (pꢀarylꢀCH), 136.7 (iꢀarylꢀC), 136.2 (oꢀarylꢀC),
Interscience Trace GC Ultra instrument using an Rxi fused silica
capillary column and pꢀxylene as internal standard.
134.9 (oꢀarylꢀC), 129.0 (CH), 128.1 (CH), 121.1 (mꢀarylꢀCH),
120.8 (mꢀarylꢀCH), 64.1 (CH2), 39.0 (alkene), 20.8 (pꢀarylꢀCH3),
17.6 oꢀarylꢀCH3, 17.5 (oꢀarylꢀCH3). MS(FAB+) for
C24H23N4O2Pd: m/z calculated 505.0865 [MꢀCO+H]+, observed
75 505.0865 .
15
General procedure synthesis bisꢀNHC palladium (0) maleic
anhydride complexes
[Pd(tBuDAB)(η2ꢀma)] (1 equiv.) was added to a solution (0.02
M) of the corresponding silver(I) complex in dichloromethane
Methylꢀ1,1ꢀdi(nButylimidazolꢀ2ꢀylidene) palladium (0) maleic
anhydride, 3e: Pale brown solid, 35 mg 0.08 mmol, 42%. 1H
20 and the mixture was stirred for 3 hours at 200C upon which the
greyish silver salt precipitated. The mixture was filtered over a
pad of Celite and concentrated to a few mL. Then pentane was
added to precipitate the product, which was isolated by decanting
the solvent and washed twice with pentane to yield the product.
25
3
NMR (CD2Cl2, 300 MHz): 7.10 (2H, d, JHH = 1.8 Hz, CH), 6.96
3
2
80 (2H, d, JHH = 1.8 Hz, CH), 5.81 (2H, dd, JHH = 12 Hz, CH2),
4.25 – 4.04 (4H, m, CH2), 3.59 (2H, s, CHꢀma), 1.85 – 1.65 (4H,
m, CH2), 1.40 – 1.22 (4H, m, CH2), 0.91 (6H, t, 3JHH = 7.3 Hz,
CH3). 13C NMR (75 MHz, CD2Cl2) 183.9 (NCN), 175.0 (CO),
119.9 (CH) , 119.8 (CH), 63.0 (CH2), 51.0 (CH2), 38.4 (alkene),
Methylꢀ1,1ꢀdi(mesitylimidazolꢀ2ꢀylidene) palladium (0) maleic
anhydride , 3a: Slightly yellow solid, 65 mg, 0.12 mmol, 80 %.
85 33.6 (CH2),
19.7 (CH2), 13.61 (CH3). MS(FAB+) for
C15H24N4Pd: m/z calculated 366.1042 [Mꢀma]+, observed
366.1053.
3
1H NMR (CD2Cl2, 300 MHz): 7.38 (2H, d, JHH = 1.5 Hz, CH),
3
7.14 (2H, s, mꢀarylꢀH), 6.96 (2H, d, JHH = 1.5 HZ, CH), 6.92
2
30 (2H, s, mꢀarylꢀH), 6.54 (1H, d, JHH = 13.2 Hz, CH2), 5.97 (d,
General procedure synthesis bisꢀNHC palladium (II) η3ꢀallyl
90 chloride complexes:
2JHH = 13.2 Hz, 1H, CH2), 2.38 (6H, s, pꢀarylꢀCH3), 2.26 (2H, s,
ma), 2.04 (6H, s, oꢀarylꢀCH3), 1.80 (6H, s, oꢀarylꢀCH3). 13C NMR
(CD2Cl2, 300 MHz): 187.0 (NCN), 173.9 (CO), 138.5 (pꢀarylꢀ
CH), 136.7 (iꢀarylꢀC), 136.2 (oꢀarylꢀC), 134.9 (oꢀarylꢀC), 129.0
35 (CH), 128.1 (CH), 121.1 (mꢀarylꢀCH), 120.8 (mꢀarylꢀCH), 64.1
(CH2), 38.4 (alkene), 20.8 (pꢀarylꢀCH3), 17.6 oꢀarylꢀCH3, 17.5
[Pd(η3ꢀallyl)Cl]2 (0.5 equiv.) was added to a solution (0.02 M) of
the silver complex in dichloromethane and the mixture was
stirred for 3 hours at room temperature upon which a greyish
silver salt precipitated. The mixture was filtered over a pad of
(oꢀarylꢀCH3). MS(FAB+) for C28H31N4O2Pd: m/z calculated 95 Celite and concentrated to yield the product.
561.1493[MꢀCO+H]+, observed 561.1487.
Methylꢀ1,1ꢀdi(mesitylimidazolꢀ2ꢀylidene) palladium (II) η3ꢀ
1
40 Ethylꢀ1,1ꢀdi(mesitylimidazolꢀ2ꢀylidene) palladium (0) maleic
anhydride, 3b: Pale brown solid, 63 mg, 0.10 mmol, 99%. 1H
allyl chloride, 4a: Off white solid, 90 mg, 0.16 mmol, 99%) . H
NMR (CD2Cl2, 300 MHz): 8.35 (2H, d,3JHH = 2.0 Hz, CH), 7.46
NMR (CD2Cl2, 300 MHz): 7.13 (2H, d, 3JHH = 1.8 Hz, CH), 7.04 100 (1H, d, JHH = 13 Hz, CH2), 6.99 (2H, d, JHH = 2.0 Hz, CH),
2
3
3
2
(2H, s, mꢀarylꢀH), 6.93 (2H, d, JHH = 1.8 Hz, CH), 6.86 (2H, s,
6.92 (4H, s, mꢀarylꢀH), 6.56 (1H, d, JHH = 13 Hz, CH2), 4.65
2
3
3
2
mꢀarylꢀH), 5.29 (2H, dd, JHH = 13.2 Hz, JHH = 6.2 Hz, CH2),
45 4.34 (2H, dd, 2JHH = 13.2 Hz, 3JHH = 6.2 Hz, CH2), 2.32 (6H, s, pꢀ
arylꢀCH3), 2.16 (2H, s, ma), 2.13 (6H, s, oꢀarylꢀCH3), 1.81 (6H, s,
(1H, m, allylꢀCH), 2.91 (2H, dt, JHH = 7.6, JHH = 1.1 Hz, allylꢀ
CH2), 2.32 (6H, s, pꢀarylꢀCH3), 1.94 (6H, s, oꢀarylꢀCH3), 1.89
(6H, s, oꢀarylꢀCH3), 1.67 (2H, dt 3JHH = 7.6, JHH = 1.1 Hz, allylꢀ
2
oꢀarylꢀCH3). 13C NMR (CD2Cl2, 300 MHz): 191.2 (NCN), 171.2 105 CH2). 13C NMR (CD2Cl2, 300 MHz): 176.6 (NCN), 139.9 (pꢀ
(CO), 138.3 (pꢀarylꢀCH), 136.7 (iꢀarylꢀC), 136.2 (oꢀarylꢀC),
134.9 (oꢀarylꢀC), 128.9 (CH), 128.1 (CH), 122.6 (mꢀarylꢀCH),
50 121.0 (mꢀarylꢀCH), 49.5 (CH2), 38.2 (alkene), 20.8 (pꢀarylꢀCH3),
17.7 (oꢀarylꢀCH3), 17.6 (oꢀarylꢀCH3). MS(FAB+) for
arylꢀCH), 137.1 (iꢀarylꢀC), 135.9 (oꢀarylꢀC), 135.6 (oꢀarylꢀC),
129.3 (mꢀarylꢀCH), 124.0 (CH), 122.1 (CH), 119.2 (allyl), 63.7
(CH2), 58.7 (allyl), 21.3 (pꢀarylꢀCH3), 18.0 (oꢀarylꢀCH3).
MS(FAB+) for C28H34N4Pd: m/z calculated 531.1751 [MꢀCl]+,
C29H34N4O2Pd: m/z calculated 504.1516 [MꢀCO+H]+, observed 110 observed 531.1757.
504.1516 .
Ethylꢀ1,1ꢀdi(mesitylimidazolꢀ2ꢀylidene) palladium (II) η3ꢀallyl
55 Methylꢀ1,1ꢀdi(methylimidazolꢀ2ꢀylidene) palladium (0) maleic
anhydride, 3c: Pale yellow solid, 64 mg, 0.17 mmol, 70%. 1H
chloride, 4b: Off white solid, 102 mg, 0.18 mmol, 92%. 1H
NMR (CD2Cl2, 300 MHz): 7.55 (2H, d, 3JHH = 1.8 Hz, CH), 7.03
3
NMR (300 MHz, CD2Cl2) δ 8.36 (2H, s, CH), 6.97 (2H, s, CH), 115 (2H, s, mꢀarylꢀH), 6.98 (2H, s, mꢀarylꢀH), 6.82 (2H, d, JHH = 1.8
2
2
6.60 (1H, d, JHH = 12.1 Hz, CH2), 5.63 (1H, d, JHH = 12.1 Hz,
HZ, CH), 5.29ꢀ5.07 (3H, m, CH2 , allylꢀCH overlapping), 4.07
Journal Name, [year], [vol], 00–00 |
This journal is © The Royal Society of Chemistry [year]
5

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2
2
(2H, d, JHH = 7.4 Hz, allylꢀCH2), 3.36 (2H, d, JHH = 5.4 Hz,
balloon with hydrogen gas and 1 mol% of catalyst, 10 mL of a
stock solution (c = 0.15 M) of 1ꢀphenylꢀ1ꢀpropyne in MeCN with
2
3
allylꢀCH2) 3.04 (2H, dd, JHH = 13.6 Hz, JHH = 6.0 Hz, CH2),
2.36 (6H, s, pꢀarylꢀCH3), 2.23 (3H, s, oꢀarylꢀCH3), 2.18 (3H, s, 60 pꢀxylene as an internal standard was added. The mixture was
oꢀarylꢀCH3), 2.08 (3H, s, oꢀarylꢀCH3), 2.02 (3H, s, oꢀarylꢀCH3).
13C NMR (CD2Cl2, 101 MHz): 181.7 (NCN), 139.5 (pꢀarylꢀCH),
136.8 (iꢀarylꢀC), 136.2 (oꢀarylꢀC), 136.0 (oꢀarylꢀC), 129.5 (CH),
129.4 (CH), 124.3 (mꢀarylꢀCH), 122.6 (mꢀarylꢀCH), 49.6 (CH2),
21.4 (pꢀarylꢀCH3), 18.7 (oꢀarylꢀCH3), 18.6 (oꢀarylꢀCH3).
MS(FAB+) for C29H35N4Pd: m/z calculated 545.1908 [MꢀCl]+,
stirred under H2 pressure at 60°C and monitored by GC.
5
Xꢀray structural determinations
Xꢀray intensities were measured on a Bruker Kappa ApexII
65 diffractometer with sealed tube and Triumph monochromator (λ =
0.71073 Å). The intensities were integrated using Saint.53
Absorption correction and scaling was performed with
SADABS.54 The structures were solved using Direct Methods in
the program SHELXSꢀ97.55 Leastꢀsquares refinement was
70 performed refined with SHELXLꢀ9755 against F2 of all
reflections. Nonꢀhydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were located in
difference Fourier maps. Hydrogen atoms of the maleic
anhydride ligand were refined freely with isotropic displacement
75 parameters, all other hydrogen atoms were refined with a riding
model. Geometry calculations and checking for higher symmetry
was performed with the PLATON program.56
10 observed 545.1914.
Methylꢀ1,1ꢀdi(methylimidazolꢀ2ꢀylidene) palladium (II) η3ꢀ
allyl chloride, 4c: Pale brown solid, 41 mg, 0.11 mmol, 63%. 1H
NMR (CD2Cl2, 300 MHz): 8.11 (2H, s, CH), 7.05 (1H, bd, 2JHH
=
15 13 Hz, CH2), 6.96 (2H, s, CH), 6.19 (1H, bd, 2JHH = 13 Hz, CH2),
2
5.26 (1H, m, allylꢀCH), 4.18 (2H, bd, JHH = 10 Hz, allylꢀCH2),
2
3.75 (6H, s, CH3), 2.85 (2H, d, JHH = 10 Hz, allylꢀCH2). 13C
NMR (CD2Cl2, 101 MHz): 175.1 (NCN), 122.9 (CH), 120.8
(CH), 119.3 (allyl), 62.4 (CH2), 58.2 (allyl), 38.5 (CH3).
20 MS(FAB+) for C12H17N4Pd: m/z calculated 323.0493 [MꢀCl]+,
observed 323.0497.
Compound 3a: Crystals suitable for Xꢀray analysis where
obtained by slow diffusion of pentane to a solution of the product
80 in dichloromethane. C29H30N4O3Pd ꢂ 0.5(CH2Cl2), Fw = 631.43,
colourless needle, 0.29 x 0.07 x 0.07 mm3, triclinic, P 1 (no. 2),
a = 11.0535(6), b = 11.5821(7), c = 11.9965(6) Å, α =
101.2168(17), β = 99.4456(17), γ = 106.8020(17) º, V =
1401.91(13) Å3, Z = 2, Dx = 1.496 g/cm3, ꢁ = 0.80 mmꢀ1. 28028
85 reflections were measured up to a resolution of (sin θ/λ)max = 0.65
Åꢀ1 at a temperature of 150(2) K. 6357 Reflections were unique
(Rint = 0.032), of which 5611 were observed [I>2σ(I)]. 366
Parameters were refined with no restraints. R1/wR2 [I > 2σ(I)]:
0.0315 / 0.0804. R1/wR2 [all refl.]: 0.0394 / 0.0845. S = 1.048.
90 Residual electron density between ꢀ1.27 and 0.90 e/Å3.
CCDC 908658 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Methylꢀ1,1ꢀdi(benzylimidazolꢀ2ꢀylidene) palladium (II) η3ꢀ
allyl chloride, 4d: Pale yellow/brown solid, 51 mg, 0.10 mmol,
1
3
25 99%. H NMR (CD2Cl2, 300 MHz): 8.06 (2H, d, JHH = 1.8 Hz,
CH), 7.52 (1H, bs, CH2), 7.45 – 7.25 (6H, m, arylꢀH), 7.21 – 7.09
3
(4H, m, arylꢀH), 6.93 (2H, d, JHH = 1.8 Hz, CH), 6.44 (1H, bs,
CH2), 5.32 – 5.19 (1H, m, allylꢀCH), 5.29 (4H, s, CH2), 4.05 (2H,
3
3
d, JHH = 6.0 Hz, allylꢀCH2), 2.78 (2H, d, JHH = 13.3 Hz, allylꢀ
30 CH2). 13C NMR (CD2Cl2, 101 MHz): 176.4 (NCN), 136.4 (arylꢀ
C), 129.6 (arylꢀCH), 128.9 (arylꢀCH), 127.8 (arylꢀCH) 123.20
(CH), 129.7 (CH), 120.2 (allyl), 63.14 (CH2), 59.34 (allyl), 55.60
(CH2). MS(FAB+) for C24H25N4Pd: m/z calculated 475.1124[Mꢀ
Cl]+, observed 475.1127..
35
Methylꢀ1,1ꢀdi(nButylimidazolꢀ2ꢀylidene) palladium (II) η3ꢀ
allyl chloride, 4e: Pale brown solid, 123 mg, 0.28 mmol, 99%.
3
1H NMR (CD2Cl2, 300 MHz) 8.01 (2H, d, JHH = 1.9 Hz, CH),
7.04 (2H, d, 3JHH = 1.9 Hz, CH), 6.12 (2H, bs, CH2), 5.38 – 5.24
95
3
40 (1H, m, allylꢀCH), 4.17 (2H, d, JHH = 7.4 Hz, allylꢀCH2), 4.09
(4H, m, CH2), 2.86 (2H, d, 3JHH = 13.2 Hz, allylꢀCH2), 1.74 (2H,
Acknowledgements
3
m, CH2), 1.44 – 1.21 (4H, m, CH2), 0.93 (6H, t, JHH = 7.4 Hz,
CH3). 13C NMR (75 MHz, CD2Cl2) 174.8 (NCN), 123.3 (CH),
122.8 (CH), 119.99 (allyl), 58.10 (allyl), 51.32 (CH2), 40.8
45 (CH2), 33.4 (CH2) , 19.7 (CH2) , 13.4 (CH3). MS(FAB+) for
C12H17N4Pd: m/z calculated 407.1434 [MꢀCl]+, observed
407.1432.
We thank the Dutch National Research School Combination
Catalysis Controlled by Chemical Design (NRSCꢀCatalysis) for
financial support within project 2009ꢀ13, Nig Pham and Basilia
100 Pires for their synthetic contribution, Han Peeters for mass
spectrometry and Jan Meine Ernsting for valuable assistance with
NMR experiments.
General procedure for catalytic transfer semihydrogenation
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(5 equiv.) and formic acid (5 equiv.) in MeCN was added. The
mixture was stirred under nitrogen at 70°C and monitored by GC.
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