Poly(benzyl ether) Dendrimers Functionalized at the Core with Palladium Bis(N-Heterocyclic Carbene) Complexes as Catalysts for the Heck Coupling Reaction

Article abstract of DOI:10.1021/acs.inorgchem.5b02629

Bis(imidazolylidene)palladium complexes 9-12 containing a sterically hindered aryl group (mesityl or 2,6-diisopropylphenyl) and a poly(benzyl ether) dendron as N-substituents of the NHC ligand are accessible up to the third generation by transmetalation o

Full text of DOI:10.1021/acs.inorgchem.5b02629

Article
pubs.acs.org/IC
Poly(benzyl ether) Dendrimers Functionalized at the Core with
Palladium Bis(N‑Heterocyclic Carbene) Complexes as Catalysts for
the Heck Coupling Reaction
Alba M. Ortiz,† Alberto Sanchez-Mendez,† Ernesto de Jesus,*,† Juan C. Flores,*,† Pilar Gomez-Sal,†
́
́
́
́
and Francisco Mendicuti‡
†Departamento de Química Organ
E28871 Alcala de Henares, Madrid, Spain
‡Departamento de Química Analítica, Química Física e Ingeniería Química, Universidad de Alcala,
́
ica y Química Inorgan
́
ica, Universidad de Alcala,
́
Edificio de Farmacia, Campus Universitario,
́
́
́
E28871 Alcala de Henares, Madrid,
Spain
S
* Supporting Information
ABSTRACT: Bis(imidazolylidene)palladium complexes 9−12
containing a sterically hindered aryl group (mesityl or 2,6-
diisopropylphenyl) and a poly(benzyl ether) dendron as N-
substituents of the NHC ligand are accessible up to the third
generation by transmetalation of the corresponding silver
complexes. Complexes 9−12 are soluble, active, and very stable
catalysts under Heck reaction conditions. The NHC ligand
appears to be stably coordinated to the Pd during catalysis. The
catalytic activity increases with generation number, although
irregularly. The palladium site is not significantly congested in the
reaction solvent by the increasing size of the dendritic
substituents, as corroborated by X-ray diffraction, fluorescence
and DOSY-NMR spectroscopy, and MD simulation studies. This
is a consequence of the conformational semiflexibility of the poly(benzyl ether) dendrons and the benzylic link between these
dendrons and the N-heterocyclic ligands.
catalytic activity, selectivity, and specificity in different
reactions.6 Moreover, their size-enlarged (or nanosized)
structure enables the possibility to run catalytic reactions in
continuous-flow membrane reactors,7,8 while their nature can
be tuned to meet the solubility requirements for the application
of other separation methodologies.8,9
INTRODUCTION
■
Advances in transition-metal-based homogeneous catalysis have
provided well-defined and highly active and selective catalysts
that can be tailored using ligand design as a key tool.1 There is,
however, an increasing need to increase the productivity of
these fine catalysts in order to make them attractive for practical
applications. Obviously, this requires the synthesis of catalysts
that are able to operate without degradationeven under harsh
conditionsand, therefore, are suitable for reuse. In terms of
coordination versatility and stability, N-heterocyclic carbenes
(NHCs) have proven to be a superb class of ancillary ligands in
modern day organometallic chemistry and have allowed the
synthesis of enhanced catalysts for a broad number of
homogeneous processes.2
Palladium dendrimers have already been tested in C−C
coupling reactions, although metal leaching and catalyst
deactivation, usually associated with the deposition of Pd
black precipitates, have been a common problem for the
recovery of the dendritic catalysts.10 We have previously studied
the behavior in this and other types of reactions of poly(benzyl
ether) or carbosilane dendrimers functionalized at the focal
point or periphery with bis- or tris(pyrazol-1-yl)methane metal
complexes.11−13 Unfortunately, the problems mentioned above
prevented reuse of the dendritic catalysts. One of the
advantages of the use of imidazole-derived NHC ligands in
Pd-mediated reactions is the strong Pd−NHC bond and
limited decomposition pathways.14 As such, we turned our
attention to dendrimer NHC complexes containing the carbene
as the anchoring ligand between the palladium center and the
Dendritic architectures3 have been used as supports for active
sites operating under homogeneous conditions.4 The bottom-
up approach to prepare this type of catalyst allows precise
control of the metal environment, while the reaction micro-
environment is progressively determined by the dendritic
structure with increasing generation, rather than by the solvent.
As a result, dendritic effects come into play in the catalytic
process.5 Catalysts enclosed in the core of dendrimers are
particularly appealing in this context since they mimic features
of enzyme catalysts and have shown remarkable effects on
Received: November 17, 2015
© XXXX American Chemical Society
A
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dendritic moiety. There are only a few examples of “formally”
real dendrimers in the literature containing rhodium,15
iridium,6 ruthenium,16 or silver17 metal complexes anchored
Scheme 1. Preparation of Dendronized NHC Precursors and
the Corresponding Silver Complexes
a
́
to the dendritic structure via a NHC ligand. In addition, Dıez-
Barra and co-workers have reported a star-shaped trimetallic
NHC Pd complex that is more active than the corresponding
monometallic catalyst in the Heck reaction between p-
bromobenzaldehyde and n-butyl acrylate.18 Additionally, Haag
and Meise have reported a highly active and water-soluble
Suzuki cross-coupling catalyst based on hyperbranched
polyglycerol decorated with bis(NHC)palladium complexes
that was reused in five consecutive reactions without a loss in
activity.19
Herein, we report the synthesis of new poly(benzyl ether)
dendrimers (Frechet dendrimers) of up to the third generation
́
containing bis(imidazolylidene)palladium complexes at their
core. The use of these macromolecules as precatalysts in a
sample Heck−Mizoroki reaction is presented. Heck coupling
usually requires high temperatures to proceed, even with
activated substrates, and therefore Pd-NHC catalysts are
particularly suitable for this reaction.20,21 The encapsulation
of palladium centers within the dendrimer interior is often
associated with enhanced stabilities,10 and even if the activity is
lowered by the encapsulation effect of the substituent, this
might be compensated by the longer durability of the catalyst.
N-Mesityl (Mes) or N-2,6-diisopropylphenyl (iPr2Ph) imida-
zoles were selected as the building blocks for the NHC
precursors because of the benefits observed in catalysis with
bulky aryl substituents on the carbene ligand.22,23 The study is
complemented with conformational and structural studies
based on solid-state X-ray structures, fluorescence techniques,
diffusion-ordered NMR spectroscopy, and molecular dynamics
simulations.
a
(a) (Gn-dend)Br, acetone, 60 °C, from 4 h for G0 to 2 days for G3.
(b) 1/2Ag2O, CH2Cl2, room temperature, from 16 h for 5a to 8 days
for 8b.
1
The H and 13C resonances of the mesityl or diisopropyl-
phenyl groups in 1−8 show the equivalence of the two halves
defined by the plane perpendicular to the ring that contains the
N−Caryl bond in solution, and the characteristic diastereoto-
picity of the two methyl groups of each isopropyl moiety in
1b−8b. The 1H resonances of the heterocycle shift to high field
after deprotonation of the imidazolium salts (1−4) and
formation of the silver complexes 5−8 (from ca. 7.0 and 7.5
to ca. 6.8 and 7.1 for H4 and H5, respectively; see Scheme 1 for
numbering scheme). The protons of the N-bonded methylene
are also shifted from ca. 5.8 to 5.3, whereas other protons of the
dendritic moiety are rather insensitive to coordination of the
NHC ligand and resonate in the three regions characteristic for
this type of poly(benzyl ether) dendron.12 With an increase in
dendron generation, the resonances of the protons located
within the dendritic core suffer a small, but steady and
consistent, upfield shift that is especially notable for the N-
bonded CH2 and H4 imidazolic protons (between −0.09 and
−0.18 ppm on going from G0 to G3 complexes). This shielding
has been interpreted as a consequence of the increasing number
of aryl groups surrounding the focal point of the dendritic
wedge in poly(benzyl ether) dendrons.12,13
RESULTS AND DISCUSSION
■
Synthesis of Dendronized (Aryl)imidazolium Salts 1−
4 and Their Silver Derivatives 5−8. The asymmetrically
N,N′-substituted imidazolium salts 1−4 were prepared in good
yields by N-alkylation of the corresponding 1-arylimidazole
compounds with the appropriate benzylic bromide dendron
(Gn-dend)Br in warm acetone (Scheme 1). The silver carbene
complexes 5−8 were obtained, also in good yields, by reaction
of the imidazolium salts with silver oxide in CH2Cl2, using Lin’s
procedure.24−26 In both synthetic steps, longer times were
required for completion of the reaction as the dendron
generation increased (see Scheme 1 and Supporting
Information for details). The preparation of 1a and 1b and
their silver derivatives 5a and 5b was previously reported using
a similar procedure.27 Compounds 1−8 were obtained as
analytically pure solids and characterized by NMR and IR
spectroscopy, and ESI mass spectrometry (see Supporting
Information for details). In addition, the crystal structures of
complexes 5a, 5b and 6b were determined by X-ray diffraction
methods (see below). The imidazolium salts 1−4 are air stable,
although hygroscopic, soluble in chlorinated and polar solvents,
and insoluble in diethyl ether and in alkanes. The silver
complexes 5−8 are also air stable, soluble in chlorinated and
polar solvents, and insoluble in alkanes. The solubility of these
compounds increases with the generation (e.g., compounds 1
are somewhat soluble in acetone whereas compounds 4 are
readily soluble) and with the iPr2Ph substituent (i.e., series b is
slightly more soluble than a, which contains a Mes group).
The carbene carbons were not detected in the 13C NMR
spectra of 5−8. This has sometimes been associated with rapid
intermolecular carbene ligand exchanges in solution via the
equilibrium [Ag(NHC) (halido)] ↔ [Ag(NHC)2][Ag-
(halido)2].24−26,28,29 This finding, together with the observation
of the [Ag(NHC)2]+ cation in the mass spectra of 5−8, agrees
with the ability of this type of complex to transfer NHC ligands.
The existence of an intermolecular ligand exchange was also
suggested by the different shapes found for the resonances of
B
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the imidazolic H4 and H5 protons, depending on the dendritic
generation and aryl substituent on the NHC ligand. These
resonances were observed as doublets for the G0 to G3 series
of complexes 5a−8a and for the G0 complex 5b (3JH,H ≈ 1.5
Hz), whereas the appearance of an additional coupling to 107Ag
or 109Ag in G1 to G3 complexes 6b−8b (both with 4JH,Ag ≈ 1.5
Hz) resulted in triplets that were better resolved for higher
generations. The triplets were converted into doublets when a
sample of 7b in CDCl3 was heated. These observations are in
agreement with an intermolecular exchange of carbene ligands
that should be expectedly slower for the most encumbered
silver centers (i.e., compounds b) and larger dendrons. The
bimolecular nature of the mechanism was supported by the
Figure 2. ORTEP representation of 5b (left; 50% probability
ellipsoids, H omitted for clarity) and mean parameters for the
Ag2(μ-Br)2 core (right). Selected bond lengths [Å] and angles [deg]
(values in brackets correspond to distances or angles in the A moiety
of the dimer): Ag(1)−C(1) 2.119(6) [2.097(5)], Ag(1)−Br(1)
2.4678(7) [2.4385(7)], Ag(1)−Br(1A) 3.0306(8) [3.173(1)],
Ag(1)···Ag(1A) 3.9729(8), C(1)−N(1) 1.349(7) [1.347(7)], C(1)−
N(2) 1.342(6) [1.341(7)], C(3)−N(2) 1.385(7) [1.383(6)], C(2)−
C(3) 1.299(8) [1.314(8)], C(2)−N(1) 1.392(7) [1.394(7)], C(4)−
N(1) 1.459(6) [1.463(7)], C(11)−N(2) 1.455(6) [1.440(6)]; Br(1)−
Ag(1)−Br(1A) 90.83(2) [88.07(3)], C(1)−Ag(1)−Br(1) 171.52(14)
[169.98(14)], C(1)−Ag(1)−Br(1A) 97.60(14) [101.3(2)], Ag(1)−
Br(1)−Ag(1A) 88.61(3) [92.51(2)], N(1)−C(1)−N(2) 105.1(4)
[104.4(4)].
1
disappearance of the H-109/107Ag coupling in more concen-
trated samples of 7b. The Imz-C2 resonance in 7b was
indirectly detected at 183.3 ppm by a 1H−13C HMBC
experiment. Interestingly, the observation of an approximate
1JC,Ag value of 250 Hz is in agreement with the predominance of
mono- over bis-NHC species in solution.30
The molecular structures determined for complexes 5a, 5b,
and 6b are shown in Figures 1−3. The metal centers in dimer
Figure 1. ORTEP representation of 5a (left; 50% probability
ellipsoids, H omitted for clarity) and parameters for the Ag2(μ-Br)2
core (right). Selected bond lengths [Å] and angles [deg] (symmetry
transformation used to generate equivalent atoms A: − x, − y, − z +
1): Ag(1)−C(1) 2.119(4), Ag(1)−Br(1) 2.6098(10), Ag(1)−Br(1A)
2.7691(10), Ag(1)···Ag(1A) 3.3403(10), C(1)−N(1) 1.352(5),
C(1)−N(2) 1.352(5), C(3)−N(2) 1.394(5), C(2)−C(3) 1.342(7),
C(2)−N(1) 1.388(6), C(4)−N(1) 1.482(5), C(11)−N(2) 1.438(5);
Br(1)−Ag(1)−Br(1A) 103.29(2), C(1)−Ag(1)−Br(1) 135.60(11),
C(1)−Ag(1)−Br(1A) 120.02(10), Ag(1)−Br(1)−Ag(1A) 76.71(2),
N(1)−C(1)−N(2) 104.2(3).
Figure 3. ORTEP representation of 6b (50% probability ellipsoids, H
omitted for clarity). Selected bond lengths [Å] and angles [deg]:
Ag(1)−C(1) 2.089(5), Ag(1)−Br(1) 2.4288(8), C(1)−N(1)
1.341(6), C(1)−N(2) 1.340(6), C(2)−N(1) 1.384(7), C(2)−C(3)
1.376(8), C(3)−N(2) 1.418(7), C(4)−N(1) 1.454(7), C(11)−N(2)
1.547(6); C(1)−Ag(1)−Br(1) 176.95(13), C(1)−N(1)−C(2)
111.6(4).
5a are situated in a distorted triangular and nearly planar
environment (bond angles ranging from 103° to 136°) and
bridged by two bromide atoms, with the N-heterocycle
approximately perpendicular to the Ag2Br2 plane (69°, Figure
1). The difference between the two Ag−Br bond distances
(2.61 and 2.77 Å) is only 0.16 Å, thus reflecting the high
symmetry of the bromido bridges in this complex compared
with other [{(NHC)Ag}2(μ-Br)2] dimeric structures reported
previously (with differences in bond distances of between 0.43
and 0.58 Å).29,31 Complex 6b is monomeric and shows a linear
arrangement around the silver(I) center, with a bonding angle
of 177.0° and an Ag−Br distance of 2.43 Å (Figure 3). The
molecular structure of 5b might be considered as an almost
frontier case between the monomeric and dimeric arrange-
ments (Figure 2). Thus, the highly asymmetric bridge in this
complex (with differences between Ag−Br distances of 0.65 Å)
can be interpreted as the result of side-on Ag···Br
intermolecular interactions at 3.10 Å between two almost
linear (NHC)AgBr monomers, with coordination parameters
(C−Ag−Br angle of 170.7° and Ag−Br distance of 2.45 Å) that
are very close to those found in 6b. The above structures are
examples of the large variety of bonding motifs that the
coordinative plasticity of silver(I) permits in the solid state with
N-heterocyclic carbene ligands and halide anions.25,32 We
recently highlighted that, even if sterically hindered NHC
ligands favor the formation of monomeric structures, a uniform
distribution of the ligand crowding around the silver-carbene
axis is also important to prevent the formation of dimers and
other aggregated structures.17 We defined the buried volume
eccentricity to parametrize the uniformity of this axial
distribution, finding a good correlation between the type of
structure adopted by (NHC)Ag(halido) complexes and the
percent buried volume (%Vbur)33 when the latter was used in
conjunction with the eccentricity parameter. For eccentricities
C
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a
of around 1.45 (as in 5a, 5b, or 6b), we predicted the
predominance of monomeric structures for NHC ligands with a
%Vbur greater than ∼30.17 The same tendency is observed in
the structures reported here, since the complex with an N-
bonded mesityl ring (5a, %Vbur = 29.4) gives a dimer, whereas
those with the most sterically demanding 2,6-diisopropylphenyl
ligand (5b and 6b, %Vbur = 33.4 and 32.6) form monomeric or
strongly distorted dimeric structures. The similar or even
slightly higher buried volume of the G0 complex 5b with
respect to the G1 analogue 6b is a consequence of the open
conformation adopted by the N-benzylic substituent in both
complexes, which moves the dendron arms away from the
metal center in 6b. For comparison, the %Vbur in complex
[Ag{1,3-(G1-dend)2-NHC)}Br], an analogue of complex 6b
with a second G1 dendron replacing the N-mesityl sub-
stituent,17 is as high as 42.2 because, in this case, the two
dendrons are folded around the metal center (the distance
between silver and one of the outer benzyl CH2 carbons is as
short as 4.12 Å, compared with 6.90 Å in 6b.
Scheme 2. Synthesis of Bis(NHC)palladium Complexes
Synthesis of Bis(carbene) Palladium(II) Complexes 9−
12. Conventional syntheses of bis(NHC) palladium(II)
complexes by deprotonation of an imidazolium salt with an
external base and reaction with a Pd(II) source, or by direct
metalation with [Pd(OAc)2], are sometimes inefficient34−36 or
lead to unpredictable results, such as the formation of
mesoionic complexes (NHC bonded via C4 or C5 instead of
C2) with ligands containing bulky N-substituents.37 Conversely,
silver NHC complexes have been shown to be excellent NHC-
transfer agents to Pd(II) in straightforward reactions.23−26 We
chose the latter as the better synthetic route to ensure
formation of the full series of G0 to G3 complexes. The most
common palladium precursors utilized for silver-mediated
carbene transfer are dichlorido or chlorido(methyl)palladium-
(II) complexes bearing weakly coordinated donor ligands such
as acetonitrile or η4-1,5-cyclooctadiene (cod).23−26,34,35,38 The
reaction of imidazolium bromides 5−8 with [PdCl2(cod)] in
dichloromethane led to mixtures of the “expected” dichlorido
complex combined with chloridobromido and dibromido
bis(carbene) complexes (Scheme 2a). The exchange of halide
between the palladium and silver centers was evidenced by the
1H NMR and ESI+ mass spectra of the crude reaction mixtures
and is favored by the higher lattice stability of AgCl versus
AgBr. It should be noted that silver bromido carbenes have
been reported to cleanly afford chloridopalladium com-
plexes,24,34,39,40 although halide scrambling has also been
described on some occasions.40−42
The mixture of palladium halide derivatives was converted
into the dibromido complexes 9−12 by refluxing the crude
reaction mixture in CH2Cl2 in the presence of an excess of LiBr
or NaBr (Scheme 2b).43 Alternatively, formation of the mixed
halide was avoided by using [PdBr2(cod)] as the metal
precursor (Scheme 2c). The bis(NHC) palladium complexes
were isolated in high yields (in general ≥90%) as analytically
pure white to pale yellow solids. They were characterized by
NMR and IR spectroscopy and ESI mass spectrometry (see
Supporting Information for details). The complexes are air
stable; soluble in chlorinated and polar solvents such as
acetone, DMF, or THF; and insoluble in alkanes. The higher
generations are also somewhat soluble in aromatic solvents,
particularly complexes 9b−12b containing the 2,6-diisopropyl-
phenyl group.
a
(a) 1/2[PdCl2(cod)], CH2Cl2, room temperature. (b) Excess of LiBr
or NaBr, refluxing CH2Cl2, 3 h. (c) [PdBr2(cod)], CH2Cl2, room
temperature, 1 h for 9a and 10a, 2 h for 11a and 12a, from 3 h for 9b
to 12 h for 12b. (d) Isomerization of trans-9a to cis-anti-9a by slow
crystallization in CH2Cl2.
1
uration is evidenced by the single H resonance observed for
the two N-bonded methylene protons of each rotamer, which
contrasts with the diastereotopicity displayed by these protons
in the cis isomers.40,42,44,45 The chemical shifts of around 170−
171 ppm for the Ccarbene are also consistent with this
assignment, since these resonances are expected at higher
field for cis arrangements (157−165 ppm).34,40,42,44 The
1
assignment of H resonances to each rotamer was based on
their relative integration and on the chemical shifts of the N-
bonded methylene protons. These protons are shielded to high
field in the anti conformation by the effect of the aromatic rings
on the NHC ligand in trans (from 6.0−5.6 ppm in the syn to
5.5−5.4 ppm in the anti rotamers, Figure 4).40,44 In the syn
conformation, shielding affects the ortho-CH3 or isopropyl CH
protons instead, which are shifted around 0.2 ppm upfield with
respect to the anti rotamers. The syn-anti equilibrium is only
slightly sensitive to the nature of the aryl substituent and,
Figure 4. Part of the 1H NMR spectrum of the mixture of syn and anti
rotamers in trans-9b.
Complexes 9−12 coexist in solution as a mixture of trans-syn
(major) and trans-anti (minor) rotamers. The trans config-
D
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interestingly, to the size of the dendritic wedge (syn/anti ratio
of around 1.9 for 9a−12a and from 2.2 to 3.0 for 9b−12b in
CDCl3 at room temperature).
Syn-anti exchange by rotation of the NHC ligand around the
carbene-palladium bond is slow on the NMR time scale at 20
1
°C, as can be seen from the sharp H resonances observed at
this temperature in CDCl3. Some broadening occurs at 55 °C,
but there is no coalescence, in agreement with the free
activation energies determined for this hindered rotation in
related nickel,40 palladium44 (ΔG‡ ≈ 75 kJ mol−1), and
platinum NHC complexes (∼85 kJ mol−1).45 Nevertheless,
chemical exchange was observed between the N-bonded
methylene protons and between the isopropyl protons of
both rotamers in a NOESY-EXSY experiment performed for 9b
in CDCl3. Moreover, the original 2:1 mixture of rotamers was
recovered immediately after the dissolution of pure samples of
trans-anti-9a or trans-syn-10a, obtained by crystallization of the
corresponding mixtures of rotamers from hexane/CH2Cl2, in
CDCl3.
Figure 6. ORTEP representation of trans-anti-9a (50% probability
ellipsoids, H omitted for clarity). Selected bond lengths [Å] and angles
[deg] (symmetry transformation used to generate equivalent atoms A:
− x, − y, − z): Pd(1)−C(1) 2.035(3), Pd(1)−Br(1) 2.4598(10),
C(1)−N(1) 1.357(4), C(1)−N(2) 1.360(4), C(2)−N(1) 1.388(4),
C(2)−C(3) 1.350(5), C(3)−N(2) 1.393(4), C(4)−N(1) 1.473(4),
C(11)−N(2) 1.455(4); Br(1)−Pd(1)−Br(1A) 180.000(7), C(1)−
Pd(1)−C(1A) 180.00(14), C(1)−Pd(1)−Br(1) 88.53(10), C(1)−
Pd(1)−Br(1A) 91.47(10), N(1)−C(1)−N(2) 105.1(3).
Cis-anti-9a was obtained by slow crystallization of the trans
mixture of rotamers 9a from CH2Cl2 (Scheme 2d). The cis
configuration is deduced from the chemical shift of the
carbenoid carbon at 163.7 ppm and the observation of two
2
doublets, at 3.35 and 6.41 ppm with JH,H = 14.1 Hz, for the
benzyl CH2 protons and two singlets, at 7.06 and 7.11 ppm, for
the meta-protons of the mesityl substituent in CDCl3 (Figure
5). The chemical inequivalence of these protons also confirms
1
Figure 5. H NMR spectrum of cis-anti-9a in CDCl3.
Figure 7. ORTEP representation of trans-syn-10a (50% probability
ellipsoids, H omitted for clarity). Selected bond lengths [Å] and angles
[deg] (values between brackets correspond to distances or angles for
the NHC ring B): Pd(1)−C(1) 2.023(3) [2.021(3)], Pd(1)−Br(1)
2.4525(7), Pd(1)−Br(2) 2.45360(7), C(1)−N(1) 1.348(4)
[1.351(4)], C(1)−N(2) 1.356(4) [1.349(4)], C(2)−N(1) 1.382(4)
[1.382(4)], C(2)−C(3) 1.345(6) [1.327(5)], C(3)−N(2) 1.400(5)
[1.399(4)], C(4)−N(1) 1.459(4) [1.467(4)], C(11)−N(2) 1.436(5)
[1.453(4)]; Br(1)−Pd(1)−Br(2) 171.68(2), C(1)−Pd(1)−C(1B)
177.61(14), C(1)−Pd(1)−Br(2) 90.29(10), C(1)−Pd(1)−Br(1)
90.39(10), C(1B)−Pd(1)−Br(1) 88.17(10), C(1B)−Pd(1)−Br(2)
91.40(10), N(1)−C(1)−N(2) 105.0(3) [104.5(3)].
the slow rotation of the NHC ligand around the palladium-
carbene bond and, as a consequence, the existence of only one
rotamer in solution (probably the anti rotamer observed in the
solid-state structure, see below). As noted by Herrmann et al.,
the rotation barrier for cis-Pd(II) complexes must be much
higher than in the trans configuration.46 These results suggest
that the formation of 9−12 as trans isomers is kinetically
controlled by the steric constraints of the bulky NHC ligands,
while the strong trans influence of these ligands favors slow
isomerization into the more thermodynamically stable cis
structure observed in the case of 9a.47
The molecular structures of trans-anti-9a and trans-syn-10a,
determined by X-ray diffraction methods, are shown in Figures
6 and 7. In addition, the structure of cis-anti-9a is shown in
Figure S1 of the Supporting Information. Although the
refinement of this structure was poor due to the low quality
of the crystal and collected data, the fundamental parameters
could nevertheless be established. The anti conformation
displayed by the NHC ligands in this cis complex is common
to all the rare examples of cis-[(NHC)2PdX2] complexes
characterized in the solid state with a sterically hindered ortho-
substituted aryl in the NHC ring.48,49 Many of the features
found in 9a and 10a are shared with other known structures of
similar palladium dihalido complexes. Thus, the mesityl
substituents are arranged almost orthogonally to the NHC
rings (72−79°), which are tilted relative to the square-planar
metal coordination plane (66−85°) to minimize steric
repulsions. The volume buried by each NHC ligand in the
G1 complex 10a (mean %Vbur of 28.1) is slightly smaller than
in the trans isomer of 9a (29.0), again reflecting the
conformational flexibility of the G1 dendrons, which are fairly
extended in 10a, with average dihedral angles at the ether
linkages (Ar−O−C−Ph) of 174°. The steric hindrance in the
syn conformation of 10a is higher in the hemispace in which the
mesityl groups are located, and this probably produces the
observed tilting of the bromido ligands to the side of the
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dendritic substituents, with a Br−Pd−Br angle of 171.7° (while
the C−Pd−C angle remains almost linear).
Evaluation of the Dendronized Pd NHC Complexes in
the Heck Reaction. The catalytic performance of complexes
9−12 was analyzed in the model Heck reaction shown in
Scheme 3 under the general conditions described in the
reaction. The reduction from 1 to 0.01 mol % in the loading of
the G1 catalyst 10a resulted in significant lowering of the
conversion rates (Figure 8b). Of more interest, however, are
the fairly similar turnover frequencies obtained at the different
concentrations (4, 6, 4, and 17 h−1, on average, after reaction
for 23 h for 1, 0.5, 0.1, and 0.01 mol % (note that the last value
is less accurate due to a higher relative error) or 13 and 12 h−1
at a fixed conversion of 20% for 1 and 0.5 mol %, respectively).
Since the reactions with complexes 9−12 occurred without
apparent degradation of the catalysts, the longevity of the active
species was assessed for the G3 complex 12a by repeated
addition of fresh substrates to the reaction medium, thus
restoring the initial concentrations once the previous catalytic
cycle was completed (24 h in DMF at 130 °C). Parallel
experiments were performed using palladium acetate under
“ligand-free” conditions or mixed with imidazolium 4a in 1:1
and 1:2 ratios. The formation of catalysts in situ by mixing
Pd(OAc)2 and a NHC precursor is a widespread practice in
similar reactions. The reactions using palladium acetate alone
or mixed with 4a all proceeded quickly with almost complete
conversion (90−100%) after 2 h in the initial reaction cycle
(cycle 0). Formation of Pd black was observed in a few minutes
for the three catalytic systems, with this formation being faster
in the absence of 4a. The reloading runs were repeated five
times for all the catalysts selected, with conversions after
reaction for 24 h that were almost quantitative and showed only
small differences between them. The conversions achieved at 2
h were more informative of the evolution of the reaction
kinetics along the consecutive cycles (Figure 9). These
Scheme 3. Model Heck Reaction Tested Using the Palladium
a
Complexes 9−12 as Catalysts
a
General conditions: methyl acrylate (0.12 M), 4-iodotoluene (0.1
M), NEt3 (0.12 M) in acetonitrile or DMF (5 mL).
caption. (E)-Methyl 4-methylcinnamate was the only product
detected by gas chromatography (GC) and 1H NMR
spectroscopy in all the tests performed. No formation of
palladium black or metal nanoparticles was detected by visual
or TEM analysis of the solutions after the reactions.
The reaction kinetics were quite reproducible (measured
conversions at 5%). As shown in Figure 8a for experiments
Figure 8. Reaction profiles for the Heck coupling in acetonitrile at 80
°C in the presence of (a) 5 μmol (1 mol %) of 9a−12a or PdCl2 and
(b) 2.5, 0.5, or 0.05 μmol (0.5, 0.1, or 0.01 mol %) of 10a. Other
conditions are summarized in Scheme 3. Conversions were
determined by periodic GC monitoring.
performed in acetonitrile at 80 °C with catalyst loadings of 1
mol %, a positive dendritic effect on the reaction kinetics was
observed on going from G0 (9a) to G3 (12a), with the
exception of 11a (G2) which, intriguingly, was the least active.
Similar activity patterns (G3 > G1 > G0 > G2) were observed
for both series of complexes (9a−12a and 9b−12b) in DMF at
130 °C using reduced Pd loadings of 0.1 mol % (Table 1).
Palladium dichloride (gray line in Figure 8a) afforded
comparable initial rates to the most active dendritic precursor
(G3); however, in contrast to 9a−12a, precipitation of copious
amounts of Pd black occurred from the beginning of the
Figure 9. Catalytic conversions (%) after 2 h in successive cycles
reloading the reaction media with fresh substrates. Pd loadings of 0.1
mol % (12a, Pd(OAc)2, and 4a + Pd(OAc)2 in molar ratios of 1:1 and
2:1) in DMF at 130 °C. Other conditions are summarized in Scheme
3.
conversions were fairly constant for 12a (13−15% in the five
runs, 35% in the initial one) with no detection of Pd(0) black
precipitates or metal nanoparticles during the full series of
cycles. The reactions using Pd(OAc)2 or 4a and Pd(OAc)2, in
which Pd black is formed in the very early stages of the initial
run, showed sharp differences depending on the catalyst
composition. Thus, under ligand-free conditions or in the
presence of 1 equiv of 4a, the rates at 2 h remained high in the
first cycles but decayed more or less abruptly in the second or
Table 1. Comparative Activities of Catalysts 9−12 in the
a
Heck Reaction
conversion (%) after reaction for 7 h
catalyst (0.1 mol %)
9 (G0)
10 (G1)
11 (G2)
12 (G3)
9a−12a
9b−12b
21
61
37
81
15
20
51
86
a
DMF at 130 °C. Other conditions as summarized in Scheme 3.
F
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third and subsequent cycles. Finally, the 2:1 mixture of
Pd(OAc)2 and 4a showed the fastest rate, with almost all
consecutive runs completed after 2 h. At this point, it should be
noted that the G3 complex 12b also showed a fairly constant
performance in these reloading experiments (around 85% after
reaction for 2 h).
It has been argued that Pd nanoparticles are formed in the
Heck reaction under ligand-free conditions and that these
soluble particles are in equilibrium with lower-order molecular
species of high reactivity, while also being prone to aggregate
further to form inactive Pd black.50,51 de Vries has extended this
mechanistic consideration to Heck reactions catalyzed at high
temperatures (120−160 °C) by precursors that are unstable
under these conditions (e.g., palladacycles, phosphine or pincer
complexes, different heterogeneous catalysts and colloids,
etc.),52 which has also been supported by strong evidence
from other groups.53 This model is consistent with the
observations made above for the reactions involving Pd(OAc)2.
After several reloading cycles, and in the absence of 4a, or at a
substoichiometric concentration thereof, the system must be
reactivated by the leaching of soluble palladium species from
larger metal particles to reach total conversion after about 24 h.
de Vries and co-workers have demonstrated that these
precipitates are readily reactivated by oxidation back to soluble
Pd(II) by I2 or Br2, which then reset the system at virtually the
same initial conditions of cycle 0.51 With Pd/4a mixtures at a
ratio of 1:2, deactivation by aggregation must be hampered to
some extent by the imidazolium salt, thus allowing the next
cycle to be catalyzed. However, the results obtained with
complexes 9−12 instead suggest the most likely involvement of
molecular active species stabilized by coordinated carbene
ligands, as proposed by Cavell et al. and Herrmann et al. for
other bis(NHC) palladium complexes,20,34,54 although un-
detected very low-order nanoparticles cannot be ruled out.55
This assumption is supported by several indicators: (a) Pd-
black or nanoparticles are not observed; (b) the catalytic
kinetics are clearly affected by the dendritic generation of the
NHC ligand (as seen for the two series of complexes in two
different solvents and temperatures); (c) the kinetics for 9−12
are quite reproducible, with TOF values that correlate well with
the concentration of Pd complex; and (d) the reactivity remains
fairly constant in successive reloading cycles.
Fluorescence Measurements. As the discontinuity in the
rate trend observed for the second-generation complexes 11a
and 11b in the Heck reaction was intriguing, we performed
several studies in an attempt to determine whether a structural
congestion of the palladium site by the poly(benzyl ether)
dendrons might be the source of this singularity. We started by
analyzing solutions of complexes 9a−12a by fluorescence
spectroscopy. The absorption spectra for these dendrimers
(Figure S2 of the Supporting Information) exhibit a typical π →
π* aromatic phenyl group band, centered at approximately 265
nm for G0 (9a). This band is shifted slightly to the red by
about ∼10 nm for the other members of the series. Another
weak band, which is only significant at relatively higher
concentrations, also appears at approximately 360 nm. The
latter band, which also becomes observable for PdBr2(cod) but
displaced to larger wavelengths (maximum at 425 nm), must be
attributed to a transition that involves a metal-to-ligand charge-
transfer state. As such, the mesitylene groups seem to exert a
strong hypsochromic effect on the emission of the Pd core.
As depicted in Figure 10, the emission spectra for 9a−12a in
CH2Cl2 upon excitation at 260 nm show a typical, and also
Figure 10. Fluorescence emission spectra for G0 to G3 dendrimers
9a−12a in CH2Cl2 at 25 °C upon excitation at 260 nm (left) and 335
nm (right).
rather weak, phenyl monomer emission band centered at ∼285
nm for 9a that shifts monotonically to the red with the
generation number. The monomer band is broadened to the
red for some of the members of the series (particularly for 10a).
This broadening, which depends strongly on the dendrimer
generation, is attributed to intramolecular excimers in the
aromatic benzyl-ether groups of the dendrons. The amount of
intramolecular excimers, as quantified by the ratio of excimers
and monomer emission intensity bands (IE/IM and other
photophysical parameters in Table S3 of the Supporting
Information), increases from 9a to 10a and decreases
monotonically from 10a through 12a. Emission upon excitation
at 335 nm for 9a−12a also exhibits a weak single fluorescence
emission band centered at ∼410 nm (Figure 10). This band is
attributed to the Pd-carbene N-heterocyclic ligand core, which
apparently behaves as a single chromophoric unit. Fluorescence
quantum yields for this emission are rather similar and are low
for all members of the series (Table S3). It seems that the size
of the dendrons has very little influence on the rate of the
deactivation processes of the Pd core. No emission at all is
observed upon excitation at 260 nm for 9a−12a in acetonitrile
or DMF, probably due to intramolecular energy transfer from
the benzyl-ether groups to the Pd core.
The fluorescence intensity decay profiles for the Pd core
emission (λex/λem = 335 nm/420 nm) were fitted to three
exponentials for any of the four 9a−12a dendrimers in CH2Cl2
at 25 °C. Components in the 0.6−1.6, 3.0−5.3, and 15.4−31.6
ns ranges were found. This indicates that other processes more
complicated than the simple radiative emission from a singlet
state are involved. Table S3 also lists weighted lifetime averages
for dilute 9a−12a/CH2Cl2 solutions at 25 °C.
Quenching of the emission from the Pd core (λex = 335 nm)
by 2,3-butanedione (diacetyl) was also studied. The resulting
Stern−Volmer plots (Figure S3 in the Supporting Information)
of the intensity measured near the maximum of Pd core
emission (420 nm) are rather linear throughout the range of
concentrations used ([diacetyl] = 0−11.7 mM; quenching
parameters obtained from the recorded plots can be found in
Table S4 of the Supporting Information). The largest
bimolecular quenching constant (kq) is observed for the first
member of the series 9a (kq ≈ 9 × 10−9 M−1 s−1). However, kq
values are smaller and rather similar, being around 4−5 × 10−9
M−1 s−1, for the rest of the members (10a−12a). The Pd core
seems to be more accessible to the quencher for 9a than for the
other members of the series (10−12). These findings also point
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to a packing of dendrons around the core, which hardly varies
for 10a−12a, as a similar quenching accessibility is observed for
these compounds in CH2Cl2.
Diffusion-Ordered NMR Spectroscopy (DOSY) Meas-
urements. As no emission was observed upon excitation at
260 nm in the solvents used in the catalytic experiments, we
tried to obtain further information by measuring the hydro-
dynamic radii in DMF-d7 by DOSY NMR.56 The translational
diffusion coefficients (D) measured at 25 °C for 9−12 in this
solvent, and the hydrodynamic radii (rh) derived from them
using the Stokes−Einstein equation, are collected in Table 2,
Table 2. Diffusion Coefficients (D), Hydrodynamic Radii
(rh), and Dendrimer Density (ρ) for 9−12 in Deuterated
DMF or Acetone at 25 °C
Figure 11. Logarithmic plots of the inverse hydrodynamic radius as a
function of molecular weight for complexes 9a−12a and 9b−12b in
deuterated acetone and DMF at 25 °C.
10
11
12
(G3)
complex
solvent
9 (G0) (G1) (G2)
DMF or acetone. Dendrimer densities decrease monotonically,
as expected for low-generation dendrimers adopting open
conformations in good solvents (Table 2).
1010 × D
9a−12a DMF-d7
5.09
3.96
9.86
3.10
7.29
2.35
5.36
a
(m2 s−1
)
acetone-
12.63
d6
Molecular Dynamic (MD) Simulations. Five-nanosecond
MD simulations in a vacuum at 300 K were performed for
trans-syn and trans-anti rotamers of the four dendrimer
generations (G0−G3) as models for dendrimers 9a−12a
following the protocols described in the theoretical background
section of the Supporting Information (Figure 12). Table 3
includes the van der Waals (VvdW) and total dendrimer
molecular volumes (VRgyr), as obtained from the van der
Waals radii and the values of the average of the root-mean-
9b−12b DMF-d7
4.83
3.84
9.15
2.95
7.26
2.16
5.43
acetone-
11.96
d6
b
rh (Å)
9a−12a DMF-d7
5.4
5.3
6.9
6.8
8.9
9.2
11.7
12.5
acetone-
d6
9b−12b DMF-d7
5.7
5.6
7.1
7.3
9.3
9.2
12.7
12.3
acetone-
d6
c
ρ (g mL−1
)
9a−12a DMF-d7
2.07
1.95
1.48
1.44
1.19
1.07
0.94
0.75
9b−12b DMF-d7
a
Measured by DOSY NMR spectroscopy at 25 °C in solutions
containing 1.0 mg of complex in 0.75 mL of deuterated solvent.
b
Hydrodynamic radii were determined from diffusion coefficients
c
using the Stokes−Einstein equation for spherical rotors. Determined
as the ratio between the hydrodynamic volume and the molar mass.
together with the values determined in acetone-d6 for the same
complexes. Acetone is considered a solvent of good quality for
poly(benzyl ether) dendrons in which the branches do not tend
to fold and collapse.57 The radii obtained are of comparable
magnitude in both deuterated solvents and increase by about
2.2 Å per generation. This generational increase is intermediate
between the value of ∼1.9 Å obtained from the radii of gyration
determined for 9a−12a in a vacuum by molecular dynamics
calculations (see below) and the increase of 2.9 Å predicted by
molecular models for fully extended structures.58 In addition, it
is quite similar to that found in didendrons with a rubicene core
in good solvents (2.3 Å per generation).57 The hydrodynamic
radii and molecular weights correlate linearly in a logarithmic
scale, with R2 coefficients for the linear regressions >0.993
(Figure 11). The slopes of the linear fits (0.50 to 0.56) are
much greater than those calculated for uncharged spherical
particles of uniform density (0.33).59 Dendrimers are neither
spherical in shape nor uniform in density; indeed, much larger
slopes are to be expected for low-generation and low-
molecular-weight dendrimers, such as those reported here,
which have a dense core, semiflexible arms, and are appreciably
nonspherical.60 In fact, similar slopes were found in poly(benzyl
ether) monodendrons having a pyrene core (0.56),58 or from
the radii of gyration determined for 9a−12a in a vacuum by the
molecular dynamic calculations studies described below (0.55).
It should be noted that no irregularities are observed in the
hydrodynamic radii of the second-generation dendrimers in
Figure 12. Most stable structures for the G3 trans-syn (upper) and
trans-anti (bottom) forms obtained from the analysis of the 5 ns
molecular dynamics trajectories. Pd and Br atoms are in magenta and
green, respectively. Notice the accessibility of the Pd core.
H
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nanoparticles is not observed, the turnover frequencies are
constant at different catalyst concentrations, and their behaviors
differ from that under “ligand-free” conditions or from mixtures
of imidazolium salts and Pd(AcO)2. In addition, the high
molecular weight (up to 3875 Da) makes some of these
dendritic catalysts suitable for recoverability studies using
nanofiltration techniques.
Table 3. van der Waals Volume (VvdW), Total Volume (VRgy),
Hydrodynamic Volume (Vh) in DMF at 25 °C, and Free
Volume Fraction (Vfree,f) for 9a−12a
a
a
b
c
dendrimer (Gn)
VvdW (nm3)
VRgyr (nm3)
Vh (nm3)
Vfree,f
9a (G0)
0.57
0.92
1.63
3.05
0.39
0.90
2.05
3.84
0.66
1.40
2.91
6.68
0.14
0.34
0.44
0.54
10a (G1)
11a (G2)
12a (G3)
The accessibility of substrates to the metal center is not
significantly constrained by the increasing size of the dendritic
substituents in complexes 9−12, as demonstrated by the
activity of these catalysts, which even improves for the larger
dendrimers. The lack of increased congestion around the
palladium site can be explained by the conformational
semiflexibility of the poly(benzyl ether) dendrons and the
benzylic link between these dendrons and the N-heterocyclic
ligands. Buried volumes determined from solid-state structures,
hydrodynamic radii obtained in solution, or quenching
experiments of fluorescence emission support this model.
Interactions other than those occurring in the closest metal
coordination sphere should, however, induce size-dependent
effects in the reactivity of metal complexes situated at the core
of dendrimers. Influences in the form of a better protection of
active sites or an increased local concentration of substrates
have been reported in the literature as one explanation for the
better rates provided, on occasions, by larger dendrons, as is the
case for those reported in this work. The discontinuity observed
in the rate trend for the G2 complexes 11 is more difficult to
explain and is not due to any particular irregularity in the
accessibility to the metal or in the molecular density of these
G2 dendrimers. As Tomalia has shown, dendritic effects are
often dictated by concurring and interdependent factors that do
not always render a property that varies regularly as a function
of only the generation.62
a
Determined from van der Waals radii and from the values of the
average of the root-mean-square radius of gyration, respectively,
obtained from the analysis of the 5 ns MD trajectories at 300 K under
the assumption of a 2:1 mixture of trans-syn and trans-anti rotamers.
b
c
Determined from the hydrodynamic radii in Table 2. Measured as
the (Vh − VvdW)/Vh ratio.
square radius of gyration calculated from an analysis of the
trajectories of the trans-syn and trans-anti rotamers under the
assumption of a 2:1 mixture. The quantitative values for both
VvdW and VRgyr are obviously smaller than the hydrodynamic
volumes (Vh) obtained experimentally in any solvent (Table 2),
and their increase with the generation number is also smaller
(see Figure S5 of the Supporting Information). This is due to
the fact that, apart from the solvation effect, the dendrons fold
more easily in a vacuum than in any solvent, thus providing
more packed structures. However, this increase is parallel to the
experimental trend obtained in good solvents for both
parameters. The free volume was also determined by
subtracting van der Waals from hydrodynamic volumes. As
expected, the fraction of the free volume increases monotoni-
cally from G0 to G3 complexes.
The radial density profiles for the centers of mass of the
phenyl ring from the dendrons and mesitylene groups (as the
number of centers per Å3), with the distance being measured
from the Pd core, were also obtained by analysis of the MD
trajectories for the trans-anti and trans-syn rotamers of the four
generations (results depicted in Figure S6 in the Supporting
Information). The data reveal a decrease in radial density with
distance to the core. The profile for G0 exhibits a maximum
located at ∼5 Å, which corresponds to the location of the
center of mass of the mesityl and phenyl groups closest to the
Pd core. The value for this maximum is smaller for the trans-
anti isomer, and the distribution slightly wider, than for the
trans-syn one. This feature also appeared for all other G1−G3
generations, with a maximum located at ∼5 Å that is wider for
the trans-anti isomers. However, another maximum was also
observed at ∼9 Å, followed by a monotonic decrease in density
with an increase in the radial coordinate. Despite the larger
steric hindrance and smaller flexibility of dendrons for the trans-
syn rotamers compared to the trans-anti ones, only a small
difference in profiles is observed between the two rotamers.
EXPERIMENTAL SECTION
The preparation of G3 compounds 4b, 8b, and 12b is given as an
example. Other experimental details can be found in the Supporting
Information.
■
Synthesis of 4b. A mixture of (G3-dend)Br63 (1.016 g, 0.6077
mmol) and 1-(2,6-diisopropylphenyl)-1H-imidazole (142 mg, 0.622
mmol) in acetone (20−25 mL) was introduced into an ampule
equipped with a PTFE valve. The tube was sealed and the mixture
stirred at 60 °C until completion of the reaction (2 days). The solvent
was then removed under a vacuum and the residue washed with cold
diethyl ether (2 × 15 mL) until 4b precipitated as a beige solid, which
was washed with pentane (yield: 1.110 g, 97%). Elemental analysis (%)
calcd for C120H111BrN2O14 (1885.07): C, 76.46; H, 5.94; N, 1.49.
Found: C, 75.89; H, 5.91; N, 1.48.
Synthesis of 8b. Ag2O (43 mg, 0.19 mmol) was added to a
Schlenk tube containing a solution of 4b (700 mg, 0.371 mmol) in
CH2Cl2 (20−30 mL) and the mixture stirred in the dark at room
temperature for 8 days. An additional amount of Ag2O (8 mg, 0.034
mmol) was added after 5 days of reaction. Once the reaction was
complete, the excess of Ag2O was separated by filtration. The solvent
was subsequently evaporated to dryness under a vacuum, and the
residue washed with hexane (3 × 20 mL) and diethyl ether (3 × 20
mL) to afford the silver complex 8b as an off-white solid (500 mg,
67%). Elemental analysis (%) calcd for C120H110AgBrN2O14
(1991.93): C, 72.36; H, 5.57; N, 1.41. Found: C, 72.50; H, 5.89; N,
1.57.
CONCLUSIONS
■
The bis(imidazolylidene)dihalidepalladium complexes 9−12,
containing a sterically hindered aryl group and a poly(benzyl
ether) dendron as N-substituents on the NHC ligand, are easily
accessible up to the third generation by transmetalation of the
corresponding silver complexes. These complexes give rise to
soluble, active, and very stable species under Heck reaction
conditions. Several observations point to homotopicity61 of the
homogeneous catalysts, with a stable coordination of the NHC
ligands. Thus, the reaction profiles are reproducible and
dependent on dendron size, the formation of Pd black or
Synthesis of 12b. [PdBr2(cod)]64 (52 mg, 0.139 mmol) and 8b
(550 mg, 0.276 mmol) were dissolved in CH2Cl2 (20−30 mL) in a
Schlenk tube, and the mixture stirred at room temperature for 12 h.
The solution was then filtered and evaporated to dryness under a
vacuum. The residue was dissolved in CH2Cl2 (20 mL) and washed
with water (3 × 15 mL). The organic layer was dried over MgSO4 and
I
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Inorganic Chemistry
Article
subsequently evaporated to dryness under a vacuum. The residue was
washed with hexane to afford 12b as a pale-yellow solid, which was
purified by column chromatography over silica gel using ethyl acetate/
hexane (1:1) as the eluent (390 mg, 73%). Elemental analysis (%)
calcd for C240H220Br2N4O28Pd (3874.55): C, 74.40; H, 5.72; N, 1.45.
Found: C, 74.51; H, 5.79; N, 1.57.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02629.
■
S
(8) van Heerbeek, R.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.;
Reek, J. N. H. Chem. Rev. 2002, 102, 3717−3756. de Jesus, E.; Flores,
́
General procedures and materials for the preparation of
the compounds and catalytic studies, characterization
data, data and background for the fluorescence measure-
ments, and the theoretical MD simulations (PDF)
Crystallographic data for 5a, 5b, trans-9a, cis-9a, and 10a
in CIF format (CIF)
J. C. Ind. Eng. Chem. Res. 2008, 47, 7968−7981.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: ernesto.dejesus@uah.es.
*E-mail: juanc.flores@uah.es.
Notes
■
́
́
dez, A.; de Jesus, E.; Flores, J. C.; Gom
́
́
ez-Sal, P.
́
chez-Men
́
dez, A.; Silvestri, G.
F.; de Jesus, E.; de la Mata, F. J.; Flores, J. C.; Gom
P. Eur. J. Inorg. Chem. 2004, 2004, 3287−3296. San
de Jesus, E.; Flores, J. C.; Gom
́
ez, R.; Gom
chez-Mendez, A.;
́
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was financially supported by the Ministerio de
■
́
chez-Mendez, A.; de Jesus, E.; Flores, J. C.; Gomez-Sal, P.
́
́
́
́
Economıa y Competitividad (CTQ2011-24096 and CTQ2014-
(14) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem.,
Int. Ed. 2007, 46, 2768−2813.
55005-P). A.M.O. is grateful to the Ministerio de Educacion
́
for
(15) Fujihara, T.; Obora, Y.; Tokunaga, M.; Sato, H.; Tsuji, Y. Chem.
Commun. 2005, 4526−4528. Fujihara, T.; Obora, Y.; Tokunaga, M.;
Tsuji, Y. Dalton Trans. 2007, 1567−1569. Sato, H.; Fujihara, T.;
Obora, Y.; Tokunaga, M.; Kiyosu, J.; Tsuji, Y. Chem. Commun. 2007,
269−271.
an FPU Doctoral Fellowship and A.S.-M. to the Universidad de
́
́
Alcala for an FPI grant. P.G.-S. is also indebted to the “Factorıa
de Crystalizacio
00015).
́
n” (Consolider-Ingenio 2010, CSD2006-
(16) Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am.
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