Palladium complexes containing potentially chelating pyridylidene-type carbene ligands

Article abstract of DOI:10.1002/ejic.200801155

Oxidative addition of 2-bromopyridine derivatives containing a potentially chelating donor group E (E = NMe2, SMe, SPh) to palladium^) gives C,￡-bound pyridylpalladium(II) complexes. Mono-, di-, and polymeric palladium complexes are obtained depending on

Full text of DOI:10.1002/ejic.200801155

FULL PAPER
DOI: 10.1002/ejic.200801155
Palladium Complexes Containing Potentially Chelating Pyridylidene-Type
Carbene Ligands
Aurélie Poulain,[a] Antonia Neels,[b] and Martin Albrecht*[a]
Keywords: Palladium / Carbene ligands / Metallacycles / Heterogeneous catalysis / Cross-coupling
Oxidative addition of 2-bromopyridine derivatives contain-
ing a potentially chelating donor group E (E = NMe2, SMe,
SPh) to palladium(0) gives C,E-bound pyridylpalladium(II)
complexes. Mono-, di-, and polymeric palladium complexes
are obtained depending on the type of functionalization at
the pyridyl nitrogen. With a lone pair at nitrogen, dimetallic
products are isolated, while protonation gives monometallic
pyridylidene-type complexes. Remarkably, N-methylation
inhibits chelating ligand coordination and a one-dimensional
polymer is formed instead. Heck-type arylation of styrene is
used as a probe for the catalytic activity of the palladium
pyridylidene complexes and reveals moderate activities.
Mechanistic studies support a heterogeneous mode of action,
including loss of the pyridylidene-type ligand from the metal
coordination sphere.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Introduction
The application of N-heterocyclic carbenes (NHCs) as
ligands for transition metals has thus far focused largely on
2-imidazolylidenes and the corresponding O- and S-deriva-
tives (oxazolylidenes and thiazolylidenes, respectively), and
their C–C saturated congeners (A–C, Scheme 1).[1] NHCs
that lack such extensive heteroatom stabilization of the
metal-bound carbon are less studied, perhaps because the
free carbene is less easily accessible.[2] Despite the pioneer-
ing work of Bertrand and co-workers,[3] who elegantly dem-
onstrated that low heteroatom stabilization does not pre-
clude the formation of stable free carbenes, protocols for
the metalation of less heteroatom-stabilized carbene ligand
precursors often avoid the formation of free carbenes.
We[4] and others[5] have recently synthesized a variety of ab-
normal and remote carbene complexes that contain NHC
ligands with heteroatoms positioned remote from the car-
bene-type carbon in reactions based, for example, on oxi-
dative addition, direct C–H bond activation (including cy-
clometalation), or transmetalation (see D–I in Scheme 1 for
examples).
Scheme 1. Different types of carbene ligands bound to transition
metals.
idylidene complexes have become accessible by a variety of
methods, predominantly[8] C–H bond activation,[9] pyridyl
alkylation,[10] and oxidative addition.[11]
Pyridylidene-type systems (G–I in Scheme 1) are a par-
ticularly attractive subclass of NHC ligands since pyridine
functionalization is highly versatile.[6] In addition, recent
work by Raubenheimer and Herrmann has demonstrated
the utility of pyridylidene complexes for catalysis.[7] Pyr-
We became interested in combining the synthetic versatil-
ity of pyridine chemistry and the catalytic activity of pyr-
idylidene-type complexes and report here on the impact of
pyridylidene ligands that incorporate chelating donor sites.
Special attention has been paid to investigating the proper-
ties and catalytic activity of the coordinated metal center as
a consequence of using different (potentially permanent
and labile) groups for nitrogen quaternization, in other
words for inducing pyridylidene-type ligand bonding.
[a] Department of Chemistry, University of Fribourg,
Chemin du Musée 9, 1700 Fribourg, Switzerland
Fax: +41-2630-09738
E-mail: martin.albrecht@unifr.ch
[b] CSEM Center Suisse d’Electronique et de Microtechnique,
Rue Jaquet-Droz 1, 2002 Neuchâtel, Switzerland
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A. Poulain, A. Neels, M. Albrecht
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Scheme 2. Synthesis of palladium pyridylidene complexes by a metalation–quaternization sequence (route B).
Results and Discussion
Synthesis of Complexes
Previously reported syntheses of chelating pyridylidene
complexes generally involve C–H bond activation strate-
gies,[9b–9f] although the heteroatom assistance in this process
Scheme 3. Product mixtures obtained upon alkylation of 2b.
may be different from that of a classical cyclometalation.[12]
In this work, we have chosen an oxidative addition protocol
in order to direct metalation selectively towards the pyr-
idine 2-position. Thus, functionalization of the meta posi-
tion of 2-bromopyridine according to known procedures[13]
afforded 2-bromo-3-(bromomethyl)pyridine (1; Scheme 2).
Subsequent nucleophilic substitution with different do-
nor groups yielded the ligand precursors 2a–c, which con-
tain a hard and basic NMe2 functionality (2a), a softer SMe
group (2b), or a mildly basic and potentially weakly coordi-
nating SPh donor site (2c) as donor group. Two conceptu-
ally different routes then exist for the fabrication of
pyridylidene-metal complexes. First, and analogous to clas-
sical imidazolium-derived NHC chemistry,[1] the pyridine
heterocycle can be transformed into the pyridinium salt by
nitrogen quaternization and subsequently be metalated
(route A). Alternatively, metalation of the pyridine may oc-
cur prior to N-functionalization (route B). With ligands 2a–
c, nitrogen alkylation (i.e., route A) appeared to be deli-
cately balanced by the basicity of the heteroatom in the do-
nor group on one hand and by the stereoelectronic effects
exerted by the bromide in the ortho position of the pyridine
nitrogen on the other. Generally, the alkylation selectivity
was low for 2a and 2b. For example, a mixture containing
the pyridinium and sulfonium salts 5b and 6, respectively,
was obtained in an approximate 4:3 ratio, along with some
dicationic species 7, when using one molar equivalent of
[Me3O]BF4 for the alkylation of 2b (Scheme 3). The alky-
lation chemoselectivity appeared to be virtually indepen-
dent of the reaction conditions (MeI in THF, MeI in
AcOH, [Me3O]BF4 in CH2Cl2/MeCN). In contrast, alky-
lation of 2c proceeded smoothly due to the low nucleophi-
licity of the phenyl-substituted sulfur, and the correspond-
ing pyridinium salt 5c was obtained in good yields (see be-
low).
yellow solids (Scheme 2). Similar pyridine-bridged rather
than halide-bridged dimeric structures have been observed
previously.[14] Complexes 3a–c are soluble in chlorinated
solvents and in polar media such as MeCN and DMSO.
Crystal structure determinations of 3a and 3b unambigu-
ously confirmed their dimeric structure (Figure 1). Both
complexes are characterized by a central six-membered di-
palladacycle in a boat-type conformation containing the
As a consequence of the low chemoselectivity of alky-
lation, route B, which involves a metal insertion–quaterniz-
ation sequence, was further pursued for the synthesis of
pyridylidene complexes. Thus, oxidative addition of ligands
Figure 1. ORTEP representation of the molecular structures of
complexes 3a (a) and 3b (b) (50% probability ellipsoids; hydrogen
atoms, co-crystallized solvent molecules, and the second crystallo-
graphically independent molecule in the asymmetric unit of 3b have
2a–c to [Pd(dba)2] provided the dimeric complexes 3a–c as been omitted for clarity).
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Palladium Complexes with Pyridylidene-Type Carbene Ligands
two palladium centers and the metal-bound carbons and
The CH2 group in the sulfur-containing complex 3b ap-
pyridine nitrogens. Additionally, two metallacycles are pears as two sets of broad AB signals at room temperature,
formed due to the E,C-bidentate coordination of the ligand. which coalesce at elevated temperatures [Tcoal = 323 K, ∆G‡
These metallacycles are puckered, with the palladium ≈ 63(Ϯ2) kJmol–1]. The AB patterns are well distinguisha-
square plane being twisted with respect to the heterocyclic ble in CD2Cl2 solution at –20 °C and reveal two isomers
pyridyl plane. The geometry around the two metal centers due to the presence of two AB patterns (2JH,H = 15.9 and
in each structure is identical, albeit not crystallographically. 13.1 Hz). The low-field part of each AB set is further split
The palladium–carbon bond lengths range from 1.951(5) into a doublet of doublets, probably because of the favor-
to 2.002(13) Å (Table 1) and do not indicate a pronounced able dihedral angle between the pyridine H4 and the equa-
double bond character due to carbene-like bonding. Car- torial proton of the methylene group. At the slow-exchange
benoid bonding may be surmised if the metal center is con- limit, rigid coordination of the sulfur donor to palladium
sidered as a (weakly) quaternizing group at nitrogen. The creates a new center of chirality and renders the methylene
ligand bite angles in both complexes 3a and 3b are identical protons diastereotopic (cf. crystal structure of 3b above).
(angle C–Pd–E: 82.1°). The crystals of 3b contain two crys- Intriguingly, coalescence of all methylene signals of 3b is
tallographically independent complex molecules with the simultaneous, thereby suggesting that inversion of the con-
two sulfur atoms in each molecule in opposite configura- figuration at sulfur is mechanistically associated with the
tions, thus providing a racemic mixture of [PdBr(pyridyl- puckering process. Accordingly, transient Pd–S dissociation
idene)] units.
may rationalize the fluxional behavior. Such a model is fur-
ther supported by the lower coalescence temperature in
DMSO as compared to CD2Cl2 solutions. The CH2 group
in complex 3c appears as a doublet located at δH = 4.14
and 5.03 ppm, respectively (500 MHz, DMSO solution).
No coalescence was observed for 3c up to 343 K, thus sug-
gesting an activation energy of more than 65 kJmol–1.
Successful cleavage of the dimers was accomplished with
methanolic HBr and produced the monometallic pyridylid-
ene complexes 4a–c. An excess of HBr may be used without
affecting the Pd–Cpyridylidene bond, thus suggesting a pro-
nounced stability of this bond. In contrast, scission of the
Pd–N bond of 4a was noted under these strongly acidic
conditions. Addition of a base such as NEt3 to the pyr-
idylidene complexes 4a–c regenerated the dimeric com-
plexes 3a–c. Formation of the monomeric complexes 4a–c
was suggested by the sharp 1H NMR resonances. Obvi-
ously, ring puckering in the monometallic complexes is fast
on the NMR time scale. Most intriguingly, the methylene
protons of 4b and 4c appear as singlets at δH = 4.43 and
Table 1. Selected bond lengths [Å] and angles [°] for complexes 3a
and 3b.
3a
3b
Pd1–C2
Pd2–C10
Pd1–N2
Pd2–N4
Pd1–N3
Pd2–N1
Pd1–Br1
Pd2–Br2
1.951(5)
1.959(5)
2.082(4)
2.091(4)
2.045(4)
2.047(4)
2.5560(6)
2.5485(6)
Pd1–C2
Pd2–C9
Pd1–S1
Pd2–S2
Pd1–N2
Pd2–N1
Pd1–Br1
Pd2–Br2
1.972(14)
2.002(13)
2.274(4)
2.265(4)
2.044(11)
2.087(11)
2.5336(19)
2.5380(15)
C2–Pd1–Br1
C2–Pd1–N2
C2–Pd1–N3
N2–Pd1–N3
N2–Pd1–Br1
N3–Pd1–Br1
169.22(13)
82.14(18)
94.00(17)
169.58(15)
94.37(12)
91.09(10)
C2–Pd1–Br1 174.6(4)
C2–Pd1–S1
C2–Pd1–N2
S1–Pd1–N2
S1–Pd1–Br1
N2–Pd1–Br1
82.1(4)
93.2(5)
170.4(3)
93.78(11)
91.4(3)
1
The H NMR spectra of the complexes (DMSO solu- 4.81 ppm, respectively, thus indicating a rapid inversion of
tions) feature broad resonances, which may be rationalized the configuration at sulfur as well. Accelerated inversion
by a conservation of the dimeric structure in solution. rates may be a consequence of the higher trans effect of
Specifically, the signal assigned to the nitrogen-bound CH2 bromide in 4 as compared to pyridine in 3.
group in 3a appears as a broad singlet at δH = 4.13 ppm
All three pyridylidene complexes 4a–c were characterized
(50 °C, DMSO solution), with a decoalescence temperature by single-crystal X-ray diffraction. Their molecular struc-
close to room temperature. In CD2Cl2 solution, the AB sig- tures are shown in Figure 2, and pertinent bond lengths and
nal (2JH,H = 14.3 Hz) is well resolved at temperatures below angles are collected in Table 2. Crystals of 4b contain two
–10 °C and coalescence occurs at around room temperature. crystallographically independent molecules in the unit cell,
This fluxional behavior presumably reflects the slow puck- with the methyl substituent in a pseudo-axial position [dihe-
ering of the five-membered N–C–C–C–Pd palladacycle. dral angle C2–Pd1–S1–C7 84.4(6)°] in both molecules. In
While this process is generally fast on the NMR timescale contrast, the sulfur substituent in 4c is in a pseudo-equato-
in related metallacycles (see below),[15] a substantial deceler- rial orientation. Both complexes 4b and 4c crystallize as
ation is expected in the dimeric structure because of pyr- racemates in the centrosymmetric space group P2l/n. The
idine coordination. As a consequence, puckering of one global features of all three structures 4a–c are very similar.
metallacycle is concerted with the puckering of the other The palladium square-plane is distorted and features Pd1–
five-membered palladacycle and includes inversion of the Br2 bonds that are consistently longer than the Pd1–Br1
boat conformation of the central dimetallacycle. Based on distances. This result is in good agreement with a higher
the resonance separation, an activation energy, ∆G‡, of trans influence of the pyridylidene carbon as compared to
54.6(Ϯ1.0) kJmol–1 was calculated for the puckering pro- the heteroatom donor. The Pd–Br1 bond length seems to
cess in 3a.
reflect the quality of the donor and is largest for soft and
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A. Poulain, A. Neels, M. Albrecht
FULL PAPER
basic SMe, decreases upon reduction of the sulfur basicity close proximity.[17] No such hydrogen bonding has been ob-
(SPh), and is smallest opposite the hard-soft most-mis- served in related palladium(II) complexes with C,N- or C,S-
matched Pd–NMe2 bond. The Pd–Cpyridylidene bond is bidentate ligands comprising a phenyl instead of a pyr-
shorter in 4a than in 4b and 4c, which is presumably a direct idylidene as the C-donor unit, despite similar torsion angles
consequence of the size of the metallacycle. The cycle in 4a between the metal coordination plane and the aryl ligand
is comparably small due to the presence of nitrogen, while [average dihedral angle of all structures 4a–c is 12(4)°].[18]
it is more expanded with sulfur. Similar differences may be
Table 2. Selected bond lengths [Å] and angles [°] for complexes
4a–c.
pointed out when comparing the structures of 3a and 3b
(see above). Intramolecular N–HN···Br1 hydrogen bonding
4a (E = N2)
4b (E = S1)
4c (E = S1)
was found to provide additional stability in all structures
4a–c. While the nitrogen-bound hydrogen atom was intro-
duced at calculated positions (N–HN 0.88 Å) in 4a and 4b,
it was located in the Fourier difference map in 4c [N–HN
0.86(4) Å]. This bonding closes a second metallacycle in the
complexes and provides an overall structure reminiscent of
pincer-type complexes.[16] The short H···Br contacts are pre-
sumably a consequence of the acidity of the nitrogen-bound
proton paired with the steric constraints imposed by the
chelating bonding mode of the pyridylidene ligand, which
predisposes the metal-bound bromide and the hydrogen in
Pd1–C2
Pd1–E
Pd1–Br1
Pd1–Br2
N1···Br1
HN···Br1
C2–Pd1–E
C2–Pd1–Br1
C2–Pd1–Br2
E–Pd1–Br1
E–Pd1–Br2
Br1–Pd1–Br2
N1–HN···Br1
Br1–Pd1–C2–N1
E–Pd1–C2–C3
1.960(2)
2.094(2)
2.4232(3)
2.5173(4)
3.174(2)
2.58
81.66(10)
91.82(7)
176.25(7)
172.54(6)
94.66(6)
91.893(11)
126
1.974 (11)
2.260 (2)
2.4662(11)
2.4908(13)
3.199(9)
2.57
84.9(3)
93.3(3)
177.84(8)
172.1(3)
93.53(4)
93.53(4)
130
1.981(5)
2.2648(12)
2.4375(7)
2.4958(8)
3.140(4)
2.54(4)
85.23(13)
92.63(12)
172.99(12)
175.22(4)
88.95(4)
93.47(3)
128(3)
8.9(2)
12.4(2)
16.0(9)
8.5(9)
12.7(4)
6.4(3)
The Pd–C bond lengths are similar to those in related
complexes containing bidentate arylamine or chelating
NHC ligands.[19] No compelling evidence is thus found for
a carbene-type Pd=C double bond based on the bonding
situation around the metal center. Inspection of the bond
lengths in the heterocycle was more instructive, in particular
when comparing the C–C distances (Table 3). All C–C
bonds in the structure of 4a are similarly long and suggest
a double bond delocalization (cf. resonance structures 4Ј
and 4ЈЈ in Scheme 4). Conversely, slight bond alternation is
noted in complex 4c, with shorter C3–C4 and C5–C6 bonds
pointing to partially localized double bonds and a more
substantial contribution of resonance structure 4ЈЈЈ. The
differences are relatively small, however, and are often
within the 3σ significance range. This is probably best illus-
trated when comparing the two molecules in the asymmet-
ric unit of complex 4b. Thus, while pyridylidene resonance
structure 4bЈЈЈ is supported by the C–C bond length alter-
nation and also by the relatively short Pd–C distance in
molecule 1, molecule 2 reveals rather delocalized double
bonds and features a comparably long Pd–C bond, in line
with a zwitterionic structure 4bЈ/4bЈЈ. Similarly, the hydro-
gen bonding in molecule 1 is slightly stronger than in mole-
Table 3. Selected heterocyclic bond lengths [Å] for complexes
4a–c.
4a
4b
4c
molecule 1 molecule 2[a]
C2–C3
C3–C4
C4–C5
C5–C6
Pd1–C2
Pd1–Br2
1.397(4)
1.387(4)
1.388(4)
1.366(4)
1.960(6)
2.5173(4)
1.420(14)
1.362(16)
1.397(14)
1.364(14)
1.974(11)
2.4908(13)
1.437(14)
1.358 (13)
1.377(14)
1.391(13)
2.002(9)
1.399(6)
1.379(7)
1.392(8)
1.352(8)
1.981(5)
2.4958(8)
Figure 2. ORTEP representation of the molecular structures of
complexes 4a (a), 4b (b), and 4c (c) (50% probability ellipsoids; the
second crystallographically independent molecule in the asymmet-
ric unit of 4b has been omitted for clarity).
2.4997(12)
[a] Atom labeling adapted.
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Palladium Complexes with Pyridylidene-Type Carbene Ligands
cule 2 (HN···Br1 = 2.55 and 2.57 Å, respectively), which is ring. In the bridging coordination mode, however, this tor-
in line with a more acidic proton and a higher iminium sion angle is maximized [angle Br1–Pd1–C2a–N1a:
character of nitrogen. Obviously, chemical differentiation 91.8(7)°], which means that the N-bound methyl group re-
(e.g., due to different substitution patterns, variation in sides above the metal coordination plane and may be in-
donor properties, chelate size) cannot account for the ob- volved in stabilizing H···Pd interactions. Short contacts
served structural changes. Accordingly, the limiting zwitter- were also observed between one of the ortho hydrogens of
ionic structures 4Ј and 4ЈЈ, and the neutral pyridylidene res- the SPh unit and palladium (H13···Pd1 2.72 Å). The pyr-
onance form 4ЈЈЈ, should be very close in energy and are idylidene and sulfur donor sites are in mutual trans posi-
probably indistinguishable in solution.
tions [angle C2a–Pd1–S1: 174.4(3)°]. The Pd1–C2a bond
length is 1.966(9) Å and hence at the shorter edge for an
unsupported Pd–C(sp2) single bond.[5q] Notably, no bond
alternation was observed in the pyridylidene heterocycle to
support a double bond-localized resonance structure sim-
ilar to 4ЈЈЈ (cf. Scheme 4).
Scheme 4. Resonance structures contributing to pyridylidene com-
plexes, featuring charge delocalization (4), charge localization in
zwitterionic species (4Ј and 4ЈЈ), and double bond localization in a
carbene-type species (4ЈЈЈ).
Alkylation rather than protonation of the pyridine nitro-
gen in the dimeric complexes 3a–c using MeI, MeOTf, or
[Me3O]BF4 has been unsuccessful thus far. Apparently,
dissociation of the Pd–Npyr bond in 3 is disfavored. As the
pyridinium ligand precursor 5c was readily available, oxi-
dative addition afforded the pyridylidene complex
8
(Scheme 5) by initial alkylation and subsequent metalation
(route A), as opposed to the sequence used for complexes
4a–c. The spectroscopic data of this complex are slightly
different from those of 4c. Thus, the CH2 protons appear
at lower field (δH = 5.02 in 8 and 4.78 ppm in 4c) and the
carbon signal of this group is at significantly higher field
(δC = 37.7 in 8 vs. 46.1 ppm in 4c). While these data are in
agreement with sulfur coordination to the metal center, they
suggest different coordination modes in 4 and 8.
Figure 3. ORTEP representation of the polymeric structures of 8
(50% probability ellipsoids; hydrogen atoms omitted for clarity).
Selected bond lengths [Å] and angles [°]: Pd1–C2a 1.966(9), Pd1–
S1 2.412(2), Pd1–Br1 2.4342(12), Pd1–Br2 2.4410(11); C2a–Pd1–
S1 174.4(3), C2a–Pd1–Br1 87.2(3), C2a–Pd1–Br2 89.3(3), S1–Pd1–
Br1 87.29(6), S1–Pd1–Br2 96.28(6), Br1–Pd1–Br2 171.26(4).
NMR Spectroscopic Analyses
Spectroscopic analyses were performed in order to evalu-
ate the effect of different functional groups at the pyridine
nitrogen, namely an alkyl group (CH3 in 8), a proton in 4,
1
and a PdL3 fragment in 3. Comparison of the H NMR
resonances provided little insight.[21] The signals due to the
metal-bound carbons appear in the 13C NMR spectrum at
around δC = 180 (for 3), 174 (for 8), and 171 ppm (for 4).
Scheme 5. Synthesis of the polymeric pyridylidene complex 8.
In the solid state, complex 8 was found to be a one-di- These resonance frequencies compare well with those of re-
mensional polymer consisting of palladium centers inter- lated pyridylidene-palladium complexes but are clearly at
linked by a bridging κ2-C,S-coordinating pyridylidene li- higher field than the carbene in the pyridylidene-platinum
gand (Figure 3). Obviously, alteration of the nitrogen sub- complex reported by Rourke and co-workers.[9a] The ob-
stituent from a proton (as in 4c, cf. Figure 2, c) to a methyl served trend may reflect the inductive effect exerted by the
group disfavors chelation, presumably because of steric re- adjacent nitrogen nucleus rather than a modification of the
pulsion between the metal-bound bromide and the N- metal–carbene bonding. Such a conclusion is supported by
bound methyl group. In coordination complexes, such re- the chemical shift differences of C6 (δC = 151 in 3, 143 in
pulsion is known to be relieved by a strongly distorted li- 8, and 140 ppm in 4), which are strongly related to those of
gand arrangement around the palladium center.[20] Chela- the metal-bound C2 nucleus. The negative inductive effect
tion of the pyridylidene ligand in 8 would induce a small of nitrogen is expected to become weaker with increasing
torsion angle between the metal square plane and the aryl donor ability of the quaternizing functional group, that is,
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A. Poulain, A. Neels, M. Albrecht
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N–PdL3 Ͼ N–H Ͼ N–CH3. The observed inversion of N– Table 4. Catalytic activity of palladium complexes 3, 8 and their
solvent homologs in the Mizoroki–Heck arylation of styrene
H and N–CH3 in this sequence may be a consequence of
the hydrogen bonding in 4.
(Scheme 6).[a]
[b]
[b]
[b]
Entry
Cat.
T [°C]
Conv.20
Conv.2h
Conv.6h
1
2
3
4
5
6
7
8
3a
140
140
140
140
140
140
140
140
100
70
140
140
140
140
13
29
32
25
21
36
22
36
2
64
86
87
71
64
90
70
79
3
97
96
95
98
98
95
98
97
56[d]
0
96
93
20
40
Catalysis
The impact of pyridylidene-type ligands on the catalytic
activity of the coordinated palladium center was probed in
the Mizoroki–Heck reaction. The arylation of styrene with
bromoacetophenone in N,N-dimethylacetamide (DMA)
was used as a model reaction (Scheme 6).
3b
3c
8
3a[c]
3b[c]
3c[c]
8[c]
3a
9
10
11
12
13
14
3a[c]
3a[c,e]
8[e]
3a[f]
8[f]
0
0
12
23
19
34
68
45
19
39
[a] General conditions: aryl bromide (1 mmol), styrene (1.5 mmol),
K2CO3 (1.1 mmol), catalyst (2 µmol, 0.2 mol-%), DMA (5 mL). [b]
Conversions after 20 min (conv.20), 2 h (conv.2h), and 6 h (conv.6h).
[c] Catalyst precursor treated with 2 mol equiv. of AgBF4 prior
to catalytic run. [d] Conversion after 24 h. [e] 8 mol-% catalyst. [f]
Addition of Hg0 (3.0 g, 15 mmol) 20 min after initiating the reac-
tion.
Scheme 6. Mizoroki–Heck reaction employed for catalyst testing.
Due to the basic conditions used for Mizoroki–Heck re-
actions, combined with the fact that bases induce rapid
transformation of complexes 4a–c into their corresponding
dimers 3a–c, catalyst screening focused on complexes 3a–c
and 8 and their solvent analogs. Initial runs indicated full
conversion after approximately 4 h when activated aryl bro-
mides were used as substrates (Table 4, entries 1–4). No
conversion was observed with aryl chlorides,[22] even after
prolonged reaction times. These results compare well with
the catalytic activity of related chelating pyridylidene-palla-
dium complexes.[23] Only small differences were noted upon
variation of the donor group E in the dimeric catalyst pre-
cursors 3a–c. Similarly, conversions with complex 8 did not
markedly differ from those obtained with the dimeric com-
plexes 3a–c (Table 4, entry 4). Abstraction of the metal-
bound halides and starting from the solvent complexes of
3 or 8 enhanced the initial turnover frequencies slightly (cf.
conversions after 20 min for entries 5–8, Table 4), although
the catalytic performance levels off to the activity of the
parent bromo complexes after this time.
High reaction temperatures are required for efficient ca-
talysis. At 100 °C, catalyst 3a gave only 56% conversion af-
ter 24 h, and no conversion at all was observed within this
time frame at 70 °C (Table 4, entries 9, 10). The need for
high temperatures, even for converting activated aryl bro-
mides, suggests that the catalysts operate heteroge-
neously.[24] The similar activities of all complexes further
strengthens this hypothesis. Moreover, higher catalyst load-
ings (Ͼ5 mol-%) did not give faster conversions than the
0.2 mol-% used in the initial runs (Table 4, entries 11, 12).
No induction time was observed when using catalyst 8,
while the catalytic activity of 3a only begins after around
10 min (Figure 4). The induction period for complex 3a
may be required for dissociation of the donor group E, a
process that has been assumed to be essential for Heck ca-
talysis using related pincer-type palladium complexes.[25]
Obviously, this step is redundant when using catalyst pre-
moidal but rather characteristic of homogeneous catalysis.
Poisoning experiments using PPh3 were not conclusive,
since the activity of the catalyst was not influenced upon
addition of 0.3 or 3 mol equiv. of phosphane. It should be
noted, however, that the catalytic system is highly sensitive
to the presence of mercury, with substrate consumption es-
sentially ceasing upon addition of excess Hg0 to an active
catalytic mixture (Table 4, entries 13, 14). This result lends
further support to a heterogeneous working mode of these
palladium pyridylidene systems.[26] Accordingly, palladium
reduction and dissociation of the pyridylidene-type ligand
constitutes a key step in entering the catalytic cycle with
complexes 3 and 8. Metal reduction to palladium(0) may
be more facile in the monomeric complex 8 than in the
dimers 3a–c, perhaps because the ligand in complex 8 con-
tains a methylated nitrogen, which ensures a more pro-
nounced pyridylidene-type ligand bonding mode. A similar
rationale may account for the higher catalytic efficiency of
the solvent complex of 8 with respect to the dibromide.
Figure 4. Time-dependent profile of the arylation of styrene cata-
cursor 8. The kinetics of the reaction are, however, not sig- lyzed by 3a (circles) and 8 (squares).
1876 Eur. J. Inorg. Chem. 2009, 1871–1881
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Palladium Complexes with Pyridylidene-Type Carbene Ligands
C8H13BrCl2N2 (288.01): calcd. C 33.36, H 4.55, N 9.73; found C
33.41, H 4.63, N 9.88.
Conclusions
A series of palladium(II) complexes containing a chelat-
ing 2-pyridylidene type ligand has been prepared. The pyr-
idine nitrogen was quaternized with a PdL3 fragment, a
proton, or an alkyl group. These different quaternizations
Synthesis of 2b: Solid NaSCH3 (0.660 g, 9.4 mmol) was added to a
solution of 1 (2.081 g, 8.3 mmol) in dry THF (20 mL) and the mix-
ture was stirred under Ar for 16 h. After addition of H2O (80 mL),
the pH of the reaction mixture was adjusted to above 11 by ad-
alter the electronic properties at the nitrogen atom and, in- dition of saturated aqueous K2CO3. The solution was extracted
with Et2O (3ϫ80 mL). The combined organic phases were washed
with H2O/brine, dried with Na2SO4, and the solvents evaporated
to dryness to yield 2b as a yellow oil (1.465 g, 81%). 1H NMR
(360 MHz, CDCl3): δ = 8.25 (dd, 3JH,H = 4.8, 4JH,H = 1.8 Hz, 1 H,
H6pyr), 7.68 (dd, 3JH,H = 7.5, 4JH,H = 1.8 Hz, 1 H, H4pyr), 7.25 (dd,
directly, also at the palladium center. Specifically, methyl-
ation of the pyridine nitrogen prevents the formation of
chelating 2-pyridylidene complexes due to steric constraints
between the N-methyl group and the ligand trans to the
chelating donor site. Heck-type arylation of styrene appears
to be slightly faster with monodentate pyridylidenes con-
taining an alkylated rather than metalated pyridine nitro-
gen. The achieved conversions suggest that thioethers do
not poison the catalytic activity of the palladium center.
Further, mechanistic studies support a heterogeneous mode
of action for this catalytic reaction. In our work, kinetic
plots erroneously support a seemingly homogeneous reac-
tion and it is obviously safer to rely on multiple mechanistic
probes than on kinetic analyses alone. The prime function
of the pyridylidene ligand in the palladium complexes
studied here seems to consist of releasing the metal center,
perhaps via reductive elimination. Such effects may need
to be taken into consideration in attempts to improve the
catalytic performance of these and related palladium pyr-
idylidene complexes.
3
3JH,H = 7.5, JH,H = 4.8 Hz, 1 H, H5pyr), 3.76 (s, 2 H, CH2), 2.05
(s, 3 H, CH3) ppm. 13C{1H} NMR (90 MHz, CDCl3): δ = 148.2
(C6pyr), 144.0 (C-Br), 138.5 (C4pyr), 135.1 (C3pyr), 122.7 (C5pyr), 37.3
(CH2), 15.2 (CH3) ppm. C7H8BrNS (218.11): calcd. C 38.55, H
3.70, N 6.42; found C 38.62, H 3.76, N 6.36.
Synthesis of 2c: A solution of BuLi (5 mL, 1.6  in hexane, 8 mmol)
was added dropwise to a solution of PhSH (1 mL, 9.8 mmol) in
dry THF (20 mL) at –78 °C. The solution was stirred at low tem-
perature for 2 h before adding a solution of 1 (2.0 g, 8 mmol) in
dry THF (3 mL). The mixture was stirred at –78 °C for a further
hour and then left to warm to room temp. After 16 h, H2O (60 mL)
was added and the pH was adjusted to above 11 with saturated
aqueous K2CO3. The mixture was extracted with Et2O (3ϫ60 mL)
and the combined organic layers were washed with H2O/brine and
dried with Na2SO4. Evaporation of all volatiles afforded 2c as a
1
yellow oil (1.944 g, 87%). H NMR (360 MHz, CDCl3): δ = 8.23
(dd, 3JH,H = 4.7, 4JH,H = 1.4 Hz, 1 H, H6pyr), 7.50 (dd, 3JH,H = 7.5,
4JH,H = 1.4 Hz, 1 H, H4pyr), 7.35–7.21 (m, 5 H, HPh), 7.14 (dd,
3
3JH,H = 7.5, JH,H = 4.7 Hz, 1 H, H5pyr), 4.14 (s, 2 H, CH2) ppm.
13C{1H} NMR (90 MHz, CDCl3): δ = 148.4 (C6pyr), 144.0 (C-Br),
Experimental Section
138.4 (C4pyr), 134.5 (C3pyr), 134.5 (CPh), 130.8 (CPh-H), 129.0 (CPh
-
General: Air-sensitive reactions were carried out under Ar using
Schlenk techniques. N,N-Dimethylformamide (DMF), N,N-di-
methylacetamide (DMA), tetrahydrofuran (THF) and CH2Cl2 were
dried by passage through solvent purification columns. 2-Bromo-
3-(bromomethyl)pyridine was prepared according to literature pro-
cedures.[13] All other chemicals are commercially available and were
used as received. 1H and 13C{1H} NMR spectra were recorded with
Bruker spectrometers at room temperature, unless stated otherwise,
and were referenced to external SiMe4. Chemical shifts (δ) are given
in ppm; coupling constants J are given in Hz. Assignments are
based either on distortionless enhancement of polarization transfer
(DEPT) experiments or on homo- and heteronuclear shift corre-
lation spectroscopy. Elemental analysis were performed by the Mi-
croanalytical Laboratory of Ilse Beetz (Kronach, Germany) and of
the ETH Zürich (Switzerland).
H), 127.1 (CPh-H), 122.7 (C5pyr), 39.0 (CH2) ppm. HR-MS (ESI):
m/z 279.97977 and 281.97717 (calcd. 279.97901 for C12H1179BrNS+
and 281.97696 for C12H1181BrNS+).
General Procedure for the Synthesis of Complexes 3a–c: Compound
2 was dissolved in dry CH2Cl2 or degassed DMSO and [Pd(dba)2]
or [Pd2(dba)3] was added. The mixture was stirred for 3 d. Subse-
quent addition of CH2Cl2 (10 mL) and Et2O (90 mL) gave a pre-
cipitate, which was collected and washed twice by addition of
CH2Cl2 and precipitation with Et2O again. The precipitate was
then dissolved in CH2Cl2 and filtered through Celite. After evapo-
ration of the solvent, complexes 3a–c were obtained as yellow pow-
ders.
Synthesis of 3a: Reaction of 2a (1.22 g, 5.67 mmol) with [Pd-
(dba)2] (3.26 g, 5.67 mmol) in CH2Cl2 (30 mL) afforded 3a (1.74 g,
48%) after purification according to the general procedure. Analyt-
Synthesis of 2a: HNMe2 (20 mL, 300 mmol) was added to an ice-
cold solution of 1 (1.509 g, 6.01 mmol) in dry THF (25 mL) and
the mixture stirred under Ar for 16 h. After addition of H2O
(70 mL), the pH of the mixture was adjusted to above 11 by ad-
dition of saturated aqueous K2CO3 and the solution was extracted
with Et2O (3ϫ70 mL). The combined organic phases were washed
with H2O/brine, dried with Na2SO4, and evaporated to give 2a as
a yellow oil (0.982 g, 76%). A microanalytically pure sample was
obtained upon recrystallization of the corresponding HCl salt
2a·2HCl from MeOH/Et2O. 1H NMR (360 MHz, CDCl3): δ = 8.26
(dd, 3JH,H = 4.6, 4JH,H = 1.7 Hz, 1 H, H6pyr), 7.77 (dd, 3JH,H = 7.7,
ically pure material was obtained by recrystallization from DMF/
3
Et2O. 1H NMR (500 MHz, DMSO, 323 K): δ = 8.54 (d, JH,H
=
5.9 Hz, 2 H, H6pyr), 7.40 (d, JH,H = 7.2 Hz, 2 H, H4pyr), 6.96 (dd,
3
3JH,H = 7.2, JH,H = 5.9 Hz, 2 H, H5pyr), 4.13 (br. s, 4 H, CH2),
3
2.80 (br. s, 12 H, CH3) ppm. 13C{1H} NMR (125 MHz, DMSO,
323 K): δ = 179.4 (C-Pd), 151.3 (C6pyr), 143.9 (C3pyr), 129.8 (C4pyr),
118.7 (C5pyr), 70.1 (CH2), 52.5 (CH3) ppm. C16H22Br2N4Pd2·H2O
(643.02): calcd. C 29.07, H 3.66, N 8.48; found C 29.01, H 4.00, N
8.84.
Synthesis of 3b: Reaction of 2b (107 mg, 0.49 mmol) with [Pd2-
3
3
4JH,H = 1.7 Hz, 1 H, H4pyr), 7.27 (dd, JH,H = 7.7, JH,H = 4.6 Hz,
(dba)3] (222 mg, 0.24 mmol) in DMSO (5 mL) according to the ge-
1 H, H5pyr), 3.51 (s, 2 H, CH2), 2.31 (s, 6 H, CH3) ppm. 13C{1H} neral procedure gave 3b (89 mg, 56%). An analytically pure sample
NMR (90 MHz, CDCl3): δ = 148.2 (C6pyr), 143.8 (C-Br), 138.7
was obtained by recrystallization from DMF/Et2O. 1H NMR
(C4pyr), 135.5 (C3pyr), 122.7 (C5pyr), 62.0 (CH2), 45.4 (CH3) ppm. (400 MHz, CD2Cl2, 253 K): δ = 8.67–8.62 (m, 2 H, H6pyr), 7.41 (m,
Eur. J. Inorg. Chem. 2009, 1871–1881
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1877

A. Poulain, A. Neels, M. Albrecht
FULL PAPER
2 H, H4pyr), 6.94–6.90 (m, 2 H, H5pyr), 4.48 (dd, 2JH,H = 15.9, 4JH,H
= 3.5 Hz, 1 H, low-field CH2 of isomer A), 4.43 (dd, JH,H = 13.1,
Synthesis of 5c: [Me3O]BF4 (0.123 g, 0.83 mmol) was added to a
solution of 2c (0.193 g, 0.68 mmol) in MeCN and CH2Cl2 (1:1,
8 mL) and the mixture stirred for 14 h. All volatiles were evapo-
rated and the residue was redissolved in MeCN (10 mL). After fil-
tration through a short pad of SiO2, volatiles were removed in
2
2
4JH,H = 4.8 Hz, 1 H, low-field CH2 of isomer B), 4.08 (d, JH,H
=
13.1 Hz, 1 H, high-field CH2 of isomer B), 3.52 (d, 2JH,H = 15.9 Hz,
1 H, high-field CH2 of isomer A), 2.95, 2.94, 2.41, 2.40 (4ϫ s, 6 H,
CH3 of isomers A and B) ppm. 13C{1H} NMR (125 MHz, DMSO,
vacuo to yield 5c as an orange waxy solid (0.195 g, 74%). 1H NMR
343 K): δ = 179.9 (C-Pd), 151.6 (C6pyr), 145.2 (C3pyr), 131.1 (C4pyr), (500 MHz, DMSO): δ = 9.12 (d, JH,H = 6.3 Hz, 1 H, H6pyr), 8.26
3
119.4 (C5pyr), 44.8 (CH2), 22.3 (CH3) ppm. C14H16Br2N2Pd2S2 (d, JH,H = 7.6 Hz, 1 H, H4pyr), 7.97 (dd, JH,H = 7.6, JH,H
=
3
3
3
(649.07): calcd. C 25.91, H 2.48, N 4.32; found C 26.17, H 2.67, N
4.03.
6.3 Hz, 1 H, H5pyr), 7.40–7.29 (m, 5 H, Har), 4.49 (s, 2 H, CH2),
4.41 (s, 3 H, NCH3) ppm. 13C{1H} NMR (125 MHz, DMSO): δ =
147.6 (C6pyr), 145.1 (C4pyr), 141.6 (C-Br), 140.4 (C3pyr), 133.2 (CPh),
130.7 (CPh-H), 129.3 (CPh-H), 127.5 (CPh-H), 125.3 (C5pyr), 51.4
(NCH3), 37.6 (CH2) ppm. C13H13BBrF4NS (382.02): calcd. C
40.87, H 3.43, N 3.67; found C 40.88, H 3.43, N 3.75.
Synthesis of 3c: The reaction of 2c (500 mg, 1.78 mmol) with
[Pd2(dba)3] (797 mg, 0.87 mmol) in DMSO (10 mL) afforded 3c as
a precipitate. An additional crop of product was isolated from the
combined supernatants of the precipitation process (total yield:
1
147 mg, 22%). H NMR (500 MHz, DMSO, 323 K): δ = 8.49 (d,
Methylation of 2b: A solution of 2b (581 mg, 2.66 mmol) in CH2Cl2
and MeCN (1:2, 12 mL) was cooled to 0 °C. [Me3O]BF4 (392 mg,
2.65 mmol) was added and the solution was stirred at 0 °C for
15 min and at room temp. for 24 h. Evaporation of an aliquot indi-
cated formation of a mixture of 2b (9%), 5b (45%), 6 (34%), and
7 (12%), according to 1H NMR integration. Solvent evaporation
and repetitive precipitation from acetone/hexane allowed for frac-
tionation and gave pure batches of 2b and 7, while mixtures of 5b
and 6 failed to separate even upon crystallization; analytical data
for these compounds were obtained from difference spectra.
3
3JH,H = 5.6 Hz, 2 H, H6pyr), 8.07 (d, JH,H = 7.4 Hz, 4 H, HPh),
3
3
7.46–7.38 (m, 8 H, 2H4 + 6HPh), 6.99 (dd, JH,H = 7.5, JH,H
=
pyr
5.6 Hz, 2 H, H5pyr), 5.03 (br. d, JH,H = 14.3 Hz, 2 H, CH2), 4.17
(br. d, 3JH,H = 14.3 Hz, 2 H, CH2) ppm. 13C{1H} NMR (125 MHz,
DMSO, 323 K): δ = 180.5 (C-Pd), 151.0 (C6pyr), 145.6 (C3pyr), 133.0
(CPh-H), 132.2 (C4pyr), 130.2 (CPh-H), 129.8 (CPh), 129.4 (CPh-H),
119.4 (C5pyr), 48.3 (CH2) ppm. C24H20Br2N2Pd2S2 (773.21): calcd.
C 37.28, H 2.61, N 3.62; found C 37.36, H 2.69, N 3.58.
3
Synthesis of 4a: Methanolic HBr (3.5 mL, 0.2  in MeOH,
0.7 mmol) was added to 3a (0.203 g, 0.31 mmol) dissolved in dry
CH2Cl2 (30 mL). After 2 d, the precipitate formed was separated
and the supernatant evaporated to dryness to give a yellow powder
(0.205 g, 82%). Recrystallization from DMF/Et2O afforded analyt-
3
5b: 1H NMR (360 MHz, MeCN): δ = 8.76 (d, JH,H = 5.9 Hz, 1
3
3
H, H6pyr), 8.37 (d, JH,H = 8.2 Hz, 1 H, H4pyr), 7.92 (dd, JH,H
=
8.2, 3JH,H = 5.9 Hz, 1 H, H5pyr), 4.37 (s, 3 H, NCH3), 3.96 (s, 2 H,
CH2), 1.96 (s, 3 H, SCH3) ppm. C8H11BBrF4NS (319.95): calcd. C
30.03, H 3.47, N 4.38; found C 30.04, H 3.42, N 4.40.
ically pure material. 1H NMR (500 MHz, DMSO, 323 K): δ =
3
13.14 (br. s, 1 H, NH), 8.35 (br. s, 1 H, H6pyr), 7.97 (d, JH,H
=
1
3
4
7.9 Hz, 1 H, H4pyr), 7.53 (t, JH,H = 6.7 Hz, 1 H, H5pyr), 4.19 (s, 2
H, CH2), 2.85 (s, 6 H, CH3) ppm. 13C{1H} NMR (125 MHz,
DMSO, 323 K): δ = 149.9 (C3pyr), 138.8 (C6pyr), 135.5 (C4pyr), 121.1
(C5pyr), 69.5 (CH2), 52.0 (CH3), C-Pd not resolved ppm.
C8H12Br2N2Pd·1/8DMF (402.42): calcd. C 24.44, H 3.15, N 7.23;
found C 24.51, H 3.18, N 6.90.
3
6: H NMR (360 MHz, MeCN): δ = 8.50 (dd, JH,H = 5.1, JH,H
= 1.4 Hz, 1 H, H6pyr), 8.11 (dd, JH,H = 7.9, JH,H = 1.4 Hz, 1 H,
3
4
H4pyr), 7.63 (dd, JH,H = 7.9, JH,H = 5.1 Hz, 1 H, H5pyr), 4.67 (s,
2 H, CH2), 2.91 (s, 6 H, SCH3) ppm. C8H11BBrF4NS (319.95):
calcd. C 30.03, H 3.47, N 4.38; found C 30.04, H 3.42, N 4.40.
3
3
1
3
4
7: H NMR (360 MHz, MeCN): δ = 8.93 (dd, JH,H = 6.3, JH,H
= 1.4 Hz, 1 H, H6pyr), 8.56 (dd, JH,H = 8.2, JH,H = 1.4 Hz, 1 H,
3
4
Synthesis of 4b: Methanolic HBr (6 mL, 0.2  in MeOH, 1.2 mmol)
was added to 3b (0.150 g, 0.23 mmol) in dry CH2Cl2 (30 mL). After
2 d, all volatiles were evaporated to give 4b as a yellow powder,
which was recrystallized from DMSO/Et2O (0.138 g, 74%). 1H
NMR (500 MHz, DMSO): δ = 13.27 (br. s, 1 H, NH), 8.44 (pseudo
t, 3JH,H = 5.8 Hz, 1 H, H6pyr), 8.10 (dd, 3JH,H = 7.7, 4JH,H = 1.0 Hz,
1H, H4pyr), 8.04 (dd, 3JH,H = 8.2, JH,H = 6.3 Hz, 1 H, H5pyr), 4.86
3
(s, 2 H, CH2), 4.41 (s, 3 H, NCH3), 2.96 (s, 6 H, SCH3) ppm.
13C{1H} NMR (100 MHz, MeCN): δ = 144.5 (C-Br), 133.7 (C3pyr),
150.6 (C6pyr), 149.4 (C4pyr), 127.9 (C5pyr), 53.2 (CH2), 47.0 (NCH3),
25.8 (SCH3) ppm. C9H14B2BrF8NS (421.79): calcd. C 25.63, H
3.35, N 3.32; found C 25.70, H 3.44, N 3.44.
3
3
4
1 H, H4pyr), 7.55 (ddd, JH,H = 7.7, JH,H = 5.8, JH,H = 1.1 Hz, 1
H, H5pyr), 4.43 (br. s, 2 H, CH2), 2.72 (s, 3 H, CH3) ppm. 13C{1H}
NMR (125 MHz, DMSO): δ = 171.0 (C-Pd), 152.7 (C3pyr), 140.0
(C6pyr), 137.9 (C4pyr), 121.2 (C5pyr), 43.9 (CH2), 23.4 (CH3) ppm.
C7H9Br2NPdS (405.45): calcd. C 20.74, H 2.24, N 3.45; found C
20.86, H 2.34, N 3.28.
Synthesis of 8: Compound 5c (0.195 g, 0.51 mmol) was dissolved
in degassed DMSO (5 mL) and [Pd2(dba)3] (0.236 g, 0.26 mmol)
was added. The mixture was stirred for 2 d, then LiBr (0.148 g,
1.7 mmol) was added and stirring was continued for another 4 h.
CH2Cl2 (8 mL) was then added and the mixture filtered through
Celite. Addition of Et2O (90 mL) induced the formation of a yellow
oil, which was separated by decantation and redissolved in MeOH
(3 mL) and CH2Cl2 (5 mL). Reprecipitation with Et2O (90 mL)
yielded 8 as a yellow powder. Another crop of product was isolated
Synthesis of 4c: Methanolic HBr (2 mL, 0.2  in MeOH, 0.4 mmol)
was added to 3c (59 mg, 0.07 mmol) in dry CH2Cl2 (15 mL). The
orange crystals which grew upon standing for 2 d were collected
and dried in vacuo (40 mg, 56%). An analytically pure sample was
obtained by recrystallization from DMF/Et2O. 1H NMR from the combined supernatants upon standing. A powder slowly
(500 MHz, DMSO, 323 K): δ = 13.33 (br. s, 1 H, NH), 8.45 (pseudo precipitated from the supernatant, which was washed with acetone
t, 3JH,H = 6.3 Hz, 1 H, H6pyr), 8.09 (dd, 3JH,H = 7.6, 4JH,H = 1.4 Hz, and dried in vacuo (combined yield: 193 mg, 78%). 1H NMR
3
3
1 H, H4pyr), 7.82–7.78 (m, 2 H, HPh), 7.53 (dd, JH,H = 7.6, JH,H
= 6.3 Hz, 1 H, H5pyr), 7.45–7.43 (m, 3 H, HPh), 4.78 (br. s, 2 H,
CH2) ppm. 13C{1H} NMR (125 MHz, DMSO, 323 K): δ = 171.4
(C-Pd), 152.5 (C3pyr), 139.8 (C6pyr), 137.4 (C4pyr), 131.4 (CPh), 130.6
(CPh-H), 129.7 (CPh-H), 129.4 (CPh-H), 121.0 (C5pyr), 46.1 (CH2)
ppm. C12H11Br2NPdS (467.52): calcd. C 30.83, H 2.37, N 3.00;
found C 30.85, H 2.47, N 2.94.
(500 MHz, DMSO, 298 K): δ = 8.60 (d, 3JH,H = 5.8 Hz, 1 H, H6pyr),
3
7.73 (d, JH,H = 7.7 Hz, 1 H, H4pyr), 7.34 (m, 3 H, 2HPh + H5pyr),
3
7.26 (t, JH,H = 7.4 Hz, 2 H, HPh), 7.16 (m, 1 H, HPh), 5.02 (br. s,
2 H, CH2), 4.74 (s, 3 H, NCH3) ppm. 13C{1H} NMR (125 MHz,
CDCl3, 298 K): δ = 173.8 (C-Pd), 143.8 (CPh), 143.7 (C6pyr), 135.0
(C4pyr), 129.1 (CPh-H), 127.3 (CPh-H), 125.7 (CPh-H), 121.4 (C5pyr),
53.2 (NCH3), 37.7 (CH2) ppm; C3pyr not resolved. C13H13Br2NPdS
1878
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1871–1881

Palladium Complexes with Pyridylidene-Type Carbene Ligands
Table 5. Crystallographic data for the reported structures.
3a
3b
4a
4b
4c
8
Color, shape
Size [mm]
Empirical formula
yellow block
0.45ϫ0.40ϫ0.35 0.45ϫ0.30ϫ0.15
C16H22Br2N4Pd· C28H32Br4N4S4Pd4· C8H12Br2N2Pd C7H9Br2NPdS
yellow plate
orange rod
yellow plate
yellow rod
yellow rod
0.45ϫ0.30ϫ0.25 0.40ϫ0.30ϫ0.20 0.23ϫ0.08ϫ0.08 0.20ϫ0.05ϫ0.05
C12H11Br2NPdS C13H13Br2NPdS
C3H7NO
716.09
193(2)
C3H7NO·C4H10O
1445.27
173(2)
Fw [gmol–1]
T [K]
402.42
173(2)
405.43
223(2)
467.50
223(2)
481.52
173(2)
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
monoclinic
P21/c (No. 14)
9.1451(8)
32.397(3)
8.8242(8)
90
115.910(10)
90
2351.6(4)
4
2.023
4.948
16396, 4594
0.0647
triclinic
monoclinic
P21/n (No. 14)
8.8792(5)
12.1669(8)
10.5364(6)
90
101.447(4)
90
1115.63(12)
4
2.396
8.784
21316, 3028
0.0381
monoclinic
P21/n (No. 14)
14.2378(17)
6.5805(5)
22.539(3)
90
92.210(14)
90
2110.1(4)
8
2.552
9.477
15548, 4081
0.1100
monoclinic
P21/n (No. 14)
9.4779(11)
13.3731(13)
10.7923(16)
90
95.903(16)
90
1360.7(3)
4
2.282
7.366
10614, 2550
0.0533
orthorhombic
Pna21 (No. 33)
13.8035(13)
11.5814(10)
8.9139(8)
90
90
90
1425.0(2)
4
2.244
P (No. 2)
¯
1
8.6224(14)
16.235(2)
17.138(3)
72.607(13)
88.346(13)
84.824(13)
2280.0(6)
2
2.105
5.277
17284, 7874
0.1031
γ [°]
V [Å3]
Z
Dcalc [g cm–3]
µ [mm–1]
Total, unique refl.
Rint
7.037
9916, 2544
0.0722
Transmission range
Parameters, restraints
R,[a] Rw[b]
GOF
0.218–0.294
269, 0
0.184–0.315
465, 0
0.0686, 0.1692
0.863
0.655–0.697
120, 0
0.0238, 0.0492
1.107
0.064–0.156
147, 0
0.0493, 0.1089
0.867
0.543–0.556
159, 1
0.0263, 0.0452
0.775
0.345–0.393
104, 1
0.0375, 0.0767
0.897
0.0348, 0.0803
0.936
Largest hole, peak [eÅ–3] –1.038, 0.859
–3.251, 2.332
–0.832, 0.440
–2.371, 1.708
–0.764, 0.502
–0.912, 1.156
2
2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo| for all I Ͼ 2σ(I). [b] wR2 = {Σw(Fo – Fc ) /Σ[w(Fo4)]}1/2
.
(481.54): calcd. C 32.42, H 2.72, N 2.91; found C 32.25, H 2.83, N
2.95.
Crystals of complex 3a contained one molecule of DMF in the
asymmetric unit, and 3b cocrystallized with one molecule of DMF
and one molecule of Et2O. The absolute structure of 8 was con-
firmed by the pertinent Flack parameters [x = 0.02(2)]. Details of
data collection and refinement parameters are collected in Table 5.
General Procedure for the Mizoroki–Heck Reaction: Bromoace-
tophenone (1 mmol), styrene (1.5 mmol), K2CO3 (1.1 mmol), and
diethylene glycol dibutyl ether (0.25 mmol) as internal standard
were dissolved in DMA (4.5 mL) and stirred at the reaction tem-
perature for 10 min before adding a DMA solution of palladium
complex (typically 0.2 µmol Pd in 0.5 mL). Aliquots were removed
at given intervals, extracted with H2O and hexane, and the organic
layer was dried with Na2SO4 and the solvents evaporated. Conver-
sions were determined by 1H NMR spectroscopy (CDCl3 solution)
using substrate/standard integral ratios.
CCDC-711206 (for 3a), -711207 (for 3b), -711208 (for 4a),
-711209 (for 4b), -711210 (for 4c), -711211 (for 8) contain the sup-
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
Solvento complexes were prepared as follows: the palladium com-
plex was dissolved in MeCN, treated with AgBF4 (2.0 mol equiv.),
and stirred for 16 h protected from light. The formed precipitate
was removed by filtration through Celite and the filtrate evaporated
to leave the corresponding solvent complex, which was used with-
out further purification.
This work was supported financially by the Swiss National Science
Foundation. M. A. gratefully acknowledges an Assistant Professor-
ship from the Alfred Werner Foundation.
Crystal Structure Determinations: Suitable single crystals were
mounted on a Stoe Mark II-Imaging Plate Diffractometer System
(Stoe & Cie, 2002) equipped with a graphite monochromator. Data
collection was performed using Mo-Kα radiation (λ = 0.71073 Å)
with a nominal crystal-to-detector distance of 70 (for 3a, 4b, and
4c), 100 (for 4a and 8), and 135 mm (for 3b). All structures were
solved by direct methods using the program SHELXS-97 and re-
fined by full-matrix least-squares on F2 with SHELXL-97.[27] The
N-bound hydrogen atom in 4c was located from a Fourier differ-
ence map and refined isotropically with a distance restraint [N–H
0.87(2) Å]. All other hydrogen atoms were included in calculated
positions and treated as riding atoms using the SHELXL-97 de-
fault parameters. All non-hydrogen atoms were refined anisotropi-
cally. A semi-empirical absorption correction was applied to all
structures using MULscanABS, as implemented in PLATON03.[28]
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1880
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1871–1881

Palladium Complexes with Pyridylidene-Type Carbene Ligands
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Received: November 28, 2008
Published Online: March 12, 2009
Eur. J. Inorg. Chem. 2009, 1871–1881
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1881