Palladium-mediated radical homocoupling reactions: A surface catalytic insight

Article abstract of DOI:10.1039/c8cy00901e

In this contribution, we report a palladium nanoparticle-promoted reductive homocoupling of haloarenes that proceeds efficiently to produce corresponding bis-aryls in moderate to excellent yields using relatively low catalyst loading (1 mol%), and exhibits broad functional group tolerance. This work sheds light on how the surface state of Pd(0) nanoparticles plays a crucial role in the reactivity of catalytic systems. Notably, the appropriate choice of palladium salts for the preparation of the preformed nanocatalysts was a key parameter having a major impact on the catalytic activity; thus, the effect of halide anions on the reactivity of the as-prepared palladium nanoparticles could be assessed, with iodide anions being capable of inhibiting the corresponding homocoupling reaction. The homocoupling reaction mechanism has been further studied by means of radical trap and electron paramagnetic resonance (EPR) experiments, revealing that the reaction proceeds via radical intermediates. Taking into account these data, a plausible reaction mechanism based on single-electron transfer processes on the palladium nanoparticle surface is discussed.

Full text of DOI:10.1039/c8cy00901e

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Palladium-mediated radical homo-coupling reactions: a surface
catalytic insight
Isabelle Favier,a Marie-Lou Toro,a Pierre Lecante,b Daniel Pla*a and Montserrat Gómez*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
In this contribution, we report a palladium nanoparticle-promoted reductive homocoupling of haloaryls that proceeds
efficiently to produce the corresponding bis-aryls in moderate to excellent yields using a relative low catalyst loading (1
mol %), and exhibiting broad functional group tolerance. This work sheds light on how the surface state of Pd(0)
nanoparticles plays a crucial role in the reactivity of catalytic systems. Notably, the proper choice of palladium salts for the
preparation of the preformed nanocatalysts was a key parameter triggereing a major impact in the catalytic activity; thus,
the effect of halide anions on the reactivity of the as-prepared palladium nanoparticles could be assessed, being iodide
anions capable to inhibit the corresponding homo-coupling reaction manifold. The homo-coupling reaction mechanism has
further been studied by means of radical trap and electron paramagnetic resonance (EPR) experiments, revealing that the
reaction proceeds via radical intermediates. Taking into account these data, a plausible reaction mechanism based on
single-electron transfer processes on the palladium nanoparticle surface is discussed.
www.rsc.org/
Introduction
In particular, we sought to explore different types of Pd-based
Catalysed homo-coupling reactions are the synthetic tools of catalysts (homogenous, heterogeneous and preformed
choice in modern organic synthesis for the synthesis of nanoparticles with defined morphology and composition) in
symmetrical bis-aryls. Recent research reports from Murphy, order to gain a thorough understanding on the catalytic
Lei, and Jiao groups on the mechanistic investigations of aryl regimes governing this transformation. In particular, improved
radical generation in “homolytic aromatic substitution” reactivity and selectivity trends have been achieved by
reactions,1, 2 evidence single-electron-transfer (SET) processes nanostructured catalysts in comparison to classical ones (both
where aryl halide radical anions are involved in the generation molecular and extended surface catalysts) in mediating the
of aryl radicals and halide anions (these reports cover transformation of organic molecules through a precise control
transition-metal free transformations).3-10 However, free aryl of the nature of catalyst.19-22 Accordingly, we envisaged further
radicals remain elusive species and even recent reports by mechanistic studies on the reaction mechanism of C(sp2)–
Studer et al. on nitroxide-mediated homo-coupling reaction of C(sp2) homo-coupling; a comprehensive understanding of the
aryl Grignard reagents suggest that this type of coupling may active species involved is of fundamental importance towards
proceed via alternative pathways.11
number of precedents concerning the Ullmann-type
a rational optimisation of the efficiency of such processes.
A
homocoupling of aryl halides and triflates catalysed by
transition metal complexes (Pd, Ni) have been described in the
literature.12-16 Notably, in situ formed Pd nanoparticles from
metal salts have been also described for chloro- and
bromoarenes in the presence of reducing agents.14, 16 In recent
years, continuous flow technological developments using
supported Pd catalysts have been successfully applied.17
Inspired by these precedents, we hypothesised that an
analogous SET process could be promoted by transition metal
nanoparticles and induce the fragmentation of aryl halides.18
Results and discussion
We focused on the reactivity and performance study of
palladium-based nanocatalysts towards the homo-coupling
reaction of haloarene substrates. The intrinsic properties of
metal nanoparticles, in particular their electronic properties
and synergistic effects among metal atoms on the surface, can
trigger particular reactivity patterns, different from reactivity
patterns provided by molecular species (monometallic
complexes or small clusters) or extended surfaces. With the
aim of studying the effect of the metal surface on the
formation of bis-aryl products, surfaces mainly modified by the
presence of anions (acetate, halides), we prepared well-
defined palladium(0) nanoparticles (PdXNPs) starting from
Pd(II) salts (where X = OAc, Cl, Br, I) in the presence of PVP as
stabiliser under hydrogen pressure, following the methodology
previously described by us (Figure 1).23,
24
This bottom-up
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synthetic protocol afforded small (less than ca. 3 nm) and well- reactions of 4’-haloacetophenones driven by hydroquinone,
dispersed nanoparticles for PdOAcNPs PdClNPs, and PdBrNPs revealing dramatic differences triggered by the presence of the
as proven by TEM, obtaining mainly spheres together with aforementioned counteranions on the reaction outcome
some anisotropic nanoparticles, most likely because anions chemoselectivity, taking into account that
,
can induce anisotropic effects on the nanoparticle growth hydrodehalogenation is a side reaction manifold (Table 1).
acting as face-specific capping agents.25-27 In the case of PdNPs prepared from Pd(OAc)2 were more active and
PdINPs, aggregation and bigger particles were observed chemoselective (entry 1, Table 1) than those prepared from
(Figure 1). The use of glycerol as solvent together with PVP PdCl2, PdBr2, and PdI2 (entries 2-4, Table 1) using 4’-
permits an efficient dispersion of the nanoparticles, avoiding bromoacetophenone as benchmark substrate. As expected, 4’-
their agglomeration and consequently preserving their chloroacetophenone was inactive using PdOAcNPs (entry 5,
reactivity; in addition, the use of H2 precludes glycerol to act as Table 1); for 4’-iodoacetophenone, the activity was slightly
reducing agent.
lower than for the corresponding bromo-arene (entry 6 vs 1,
Table 1), accordingly to a higher adsorption of iodoarenes than
bromoarenes on the palladium surface;32 but in this case, the
reaction was not selective, obtaining up to 40% of
acetophenone. These results evidence the dramatic role of
small amounts of iodides and chlorides (0.071 mol% and 0.023
mol% respectively) in the reaction medium (entries 2 and 4,
Table 1), and the less severe effect of bromide (entry 3, Table
1), but also significant in comparison with PdOAcNPs used as
catalyst (entry 3 vs 1, Table 1). It is important to note that the
homo-coupling reaction was easily scaled up (entry 7 vs 1,
Table 1), obtaining the same catalytic behaviour than at
smaller scale.
PdI2
H2 (3 bar)
PdBr2
PdCl2
O
n
PdXNPs
N
+
glycerol
80 °C, 12 h
X = I, Br, Cl, OAc
Pd(OAc)2
PVP
Monomer/Pd = 20
Table 1. Preformed PdNPs as catalysts in the homo-coupling reaction of 4’-
haloacetophenones.a
HO
OH
O
(1 equiv.)
Me
PdNPs (1 mol%) in PVP
KOH (1 equiv.)
X
glycerol, 90 ºC, 18 h
+
O
O
O
Me
Me
Me
Figure 1. Synthesis of PdNPs stabilised by PVP in glycerol from Pd(II) salts and the
1, X = Br
4
5
corresponding TEM images from the corresponding glycerol colloidal solutions.
2
, X = Cl
3, X = I
PdNPs were isolated at solid state from glycerol solutions by
centrifugation. Powder X-ray diffraction analyses proved that
the as-prepared nanoparticles exhibit a fcc crystalline structure
(see Fig. S1 in the ESI) and the XPS analysis of PdOAcNPs
evidenced the presence of Pd(0) and the absence of Pd(II) (see
Fig. S2 in the ESI). In agreement with these data, Raman
conv. selectivity yield of 4
entry substrate
PdNPs
(%)b
(4:5)b
(%)b
1c
2
1
1
1
1
2
3
1
PdOAcNPs
PdClNPs
68
9.0:1.0
61
4
n.d.
9.0:1.0
n.d.
n.d.
20
3
PdBrNPs
PdINPs
22
3
spectra of PdNPs in the region corresponding to Pd-X
stretchings (400-200 cm-1)28,
4
n.d.
n.d.
41
29
did not show any band,
5
PdOAcNPs
PdOAcNPs
PdOAcNPs
<1
66
74
n.d.
indicating that no Pd-X covalent interactions are present in
these materials (see Fig. S3 in the ESI); for PdOAcNPs, no
absorption bands were observed in the region 1700-1300 cm-1,
pointing that acetates are not adsorbed at the surface.
However, halides can be just adsorbed at the surface, due to
their high adsorption enthalpies on palladium surface.30
Actually, X-ray fluorescence analyses revealed the presence of
halides on the PdNPs as follows: Cl/Pd = 1/42 (2.3%), Br/Pd =
1/36 (2.7%), and I/Pd = 1/13 (7.1%), showing a significant
higher amount of iodide on the nanoparticles than chlorine or
bromine, according to the high affinity of iodide to
palladium.31
6
6.0:4.0
9.0:1.0
7d
68e
a
Reactions carried out under the general experimental procedure (see ESI for
b
details). Determined by GC-MS after 18 h of reaction time unless otherwise
c
stated (tetrachloroethane was used as internal standard). No reaction in the
d
absence of catalyst. Results obtained at large scale (based on 5 mmol of
substrate instead of 1 mmol used under the standard conditions applied for the
other entries). e Isolated yield.
X-fluorescence analysis of PdOAcNPs isolated at solid state
after catalytic reaction (entry 1, Table 1), showed the presence
of bromine at a relative high content, namely a Br/Pd ratio of
1/18, which represents twice the amount of bromine present
in the preformed palladium nanoparticles from PdBr2 (see
The reactivity profiles using these preformed PdNPs were then
investigated towards reductive carbon–carbon homo-coupling
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above). Consequently, the catalytic activity of PdOAcNPs is the enhancement of the reactivity profiles, in the presence of
probably hampered due to the adsorbed bromide ions, coming halide additives for the oxidative homo-coupling of boronic
from the substrate, at the metal surface, triggering a poisoning acids catalysed by palladium nanoparticles, suggests a leaching
effect.
of palladium to the solution promoted by halides,40 in contrast
The observed reactivity trends depending on the nature of the to our results.
anions present in solution correlates nicely with the To validate the hypothesis of the deleterious halide effect on
Hofmeister series,33 the known capability of ions to perturbate the nanoparticle surface, we envisioned using a strong base
the hydrogen-bonded structure of bulk water,34-37 which may supported on a cationic resin to withdraw halide ions from the
play a similar role to analogous 3D structured solvents by reaction medium via ionic interaction with the polymeric
hydrogen bonding interactions such as glycerol. Namely, the beads. For this purpose, we chose the Amberlite IRA-900
Hofmeister effect for the involved anions corresponds to the hydroxide
form
featuring
trimethyl
ammonium
following trend: CH3COO- > Cl- > Br- > I-, what means that functionalisation.41 However, the use of this cationic resin in
acetate anions are better solvated by glycerol than halides and the presence of KOH did not trigger any significant effect
in consequence they are not directly interacting with the towards the conversion of 4’-bromoacetophenone (entry 2 in
surface. This behaviour also correlates with the better Table S2 vs entry 1 in Table 1). Moreover, the addition of
adsorption of halides, in particular iodide, at the surface.38
NaOAc in the homo-coupling of 4’-bromoacetophenone using
The optimisation of the reaction conditions was carried out PdOAcNPs or PdINPs as catalyst, did not improved the
(temperature, base, and additives) for 4’-bromoacetophenone, reactivity, evidencing that acetate anions do not modify
being KOH (as base), hydroquinone (as reducing agent) and a significantly the metal surface (entries 4 and 5 in Table S2 in
catalyst loading of 1 mol% (total palladium) the optimal the ESI).
conditions for the synthesis of 4,4’-bisacetophenone (see The scope of the reaction was further studied towards the
Table S1 in the ESI). Under base-free conditions, the reaction preparation of bis-aryls from moderate to good yields; for
did not proceed (entry 1 in Table S1). Thus, four different conversions lower than 15%, the homocoupling product was
bases were evaluated (namely, NaOH, KOH, Na2CO3, and NEt3; not isolated (Figure 2). As expected, we observed
entries 2-5 in Table S1). The results showed that strong bases combination of both inductive and steric effects, in particular
such as NaOH and KOH were more efficient for the homo- with preference for the substrates bearing electron-
coupling. In terms of reaction additives, the role of attracting and low steric-demanding groups in para position,
hydroquinone proved rather essential, not only to boost respectively. High chemoselectivity of bis-
a
a
reaction conversion towards the homo-coupling product, but aryl:hydrodehalogenation product (up to 9:1) was obtained for
also to preclude substrate hydrodehalogenation (entry 6 vs 3 all substrates tested unless those containing a bulky group in
in Table S1). Hydroquinone was then tested at different ortho position (15, 16), electron-withdrawing groups in both
amounts (from 0 to 6 equiv.),39 being the optimal amount 1 meta positions (11) or even a combination of both stereo-
equiv. (entries 3 and 7-9 in Table S1). At 120 °C, no significant electronic factors led to inverse the general selectivity trends
changes were observed in relation to the behaviour observed
(
16). Besides, the low conversions obtained for substrates
14) may be due to a plausible
at 90 °C (entries 3 and 10 in Table S1). However, a remarkable containing nitrile groups (
conversion decrease was observed upon recycling of the catalyst poisoning.
catalytic phase (entry 11 in Table S1). TEM analysis after
catalysis showed the presence of different populations of
nanoparticles (from ca. 1.5 to 14 nm; see Fig. S4 in the ESI),
which can explain the loss of reactivity. This fact can be
probably related to the Ostwald ripening due to palladium
leaching from the nanoparticle surface promoted by halides.
The presence of molecular species in the catalytic medium is
most likely responsible of the hydro-dehalogenation by-
product (see below, Table 2)
9,
Concerning the use of other additives, and in particular halide
salts, we realised that the addition of KI (0.2 equiv.) to the
reaction medium hampered the conversion of 4’-
bromoacetophenone even in the presence of base (9%
conversion; see entry 1 in Table S2 in the ESI), which correlates
to the experiments where using PdNPs prepared from PdI2
(entry 4 in Table 1). This clearly demonstrates the deleterious
effect of iodides for this type of Pd-based nanocatalysts,
notably due to the adsorption of iodide anions at the surface,
in agreement with the trend observed by Liu, Xia and co-
Figure 2. Reaction scope of PdOAcNPs catalysed homo-coupling reaction of
workers in the hydrodehalogenation of halogenated aromatic
compounds.38 However, a recent report by Buurma et al. on
bromoarenes. Isolated yields given in brackets; conversion and selectivity (bis-
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aryl:hydrodehalogenation ratio) given in square brackets (determined by GC-MS using
tetrachloroethane as internal standard). a Yield determined by GC-MS.
At this point, the need of direct comparison of nanocatalytic
systems with both classical homogeneous and heterogeneous
regimes described for C-C coupling reactions became
mandatory. For this purpose and concerning the homogeneous
catalytic system, we envisaged the use of the Herrmann-Beller
palladacycle (cataCXium® C) as a source of stable Pd(II)
molecular species (entries 1-2, Table 2). In toluene, the
complex was inactive (entry 1, Table 2), while in glycerol, the
conversion was of 62%, but favouring the formation of the
hydrodehalogenation product (bis-aryl:acetophenone = 1:4).
Furthermore, molecular catalytic species generated in situ
from the corresponding metallic salts (Pd(OAc)2, PdCl2, PdBr2,
PdI2) in the presence of PPh3 were also studied (entries 3-6,
Table 2). Despite PdCl2, PdBr2, and PdI2 gave negligible
amounts of the bis-aryl product (entries 4-6, Table 2), the use
of Pd(OAc)2 under analogous conditions gave the bis-arylation
product, but in moderate yield (44%; entry 3, Table 2), due to
Figure 3. TEM micrographs in glycerol of cataCXium®
C under catalytic reaction
conditions (entry 2, Table 2): after 15 minutes (left) and 18 h of reaction (right).
For comparison purposes with a heterogeneous palladium
catalyst, we used Pd/C (10 wt % metal loading on matrix
activated carbon support) constituted by aggregates together
with small nanoparticles (dmean = 1.7±0.4 nm; see Fig. S5 in the
ESI), in the benchmark reaction. The catalyst was highly active
(>99% conversion after 18 h and 73% conversion after 6 h), but
the hydrodehalogenation by-product was formed in high
proportion (30%), even at shorter times (entry 13, Table 2).
Table 2. Pd-based catalytic controls in the homo-coupling reaction of 4’-
bromoacetophenone.a
the
important
formation
of
acetophenone
(bis-
aryl:acetophenone = 3:2). In the absence of PPh3 (entries 7-9,
Table 2), yields in bis-aryl were higher (entries 7-9 vs 3-5, Table
2), being Pd(OAc)2 the most active catalytic system
(Pd(CF3COO)2 led to a similar catalytic trend; entry 10, Table 2).
Analogously to cataCXium® C, these catalysts were inactive in
toluene (entries 11 and 12, Table 2). This behaviour highlights
the ability of glycerol to act as a reducing agent of Pd(II)
HO
OH
O
(1 equiv.)
Me
Pd (1 mol%)
KOH (1 equiv.), additive
solvent, 90 ºC, 18 h
Br
+
O
O
O
Me
Me
Me
1
4
5
yield
of 4
(%)b
-
conv. selectivity
entry
Pd catalyst
additive solvent
precursors towards the formation of Pd(0) species (polyol
43
(%)b
(4:5)b
methodology synthesis of metal nanoparticles),42,
agreement with a recent report from Kapdi and coworkers.44
in
1
2
3
cataCXium® C
cataCXium® C
Pd(OAc)2
-
-
PhMe
glycerol
glycerol
0
-
62
75
2.0:8.0
6.0:4.0
12
These results prompted us to further study whether the
catalysts involved in such control tests had underwent any
PPh3
(2 mol%)
PPh3
(2 mol%)
PPh3
(2 mol%)
PPh3
(2 mol%)
-
44
significant modifications throughout the reaction as
a
4
5
6
PdCl2
PdBr2
PdI2
glycerol
glycerol
glycerol
7
5
5.5:4.5
2.0:8.0
-
4
n.d.
-
suspicion that some sort of palladium clusters or NPs might
have formed under catalytic conditions. Thus, aliquots were
taken at two different reaction times (after 15 min and 18 h of
reaction) for the cataCXium® C catalytic system. Notably, the
hydrodehalogenation by-product acetophenone could be
detected in less than 1% by GC-MS after 15 min reaction time
(conversion less than 5%), without any traces of the
corresponding bis-aryl product. However, TEM analyses
(carried out in glycerol solution due to the negligible vapour
pressure of this solvent) of the catalytic phase just after 15 min
of reaction revealed the formation of small and well-dispersed
spherical nanoparticles (Figure 3).45 These results indicate that
cataCXium® C is a precursor of a palladium nanocatalyst
responsible of the observed reaction outcomes. The low
conversion at short times only giving acetophenone, suggests
that this by-product is probably formed from molecular
catalytic species. After 18 h of reaction, nanoparticles are less
homogeneous in size with the formation of some aggregates,
probably due to the lack of a suitable stabilising agent in the
medium.
<3
7
8
Pd(OAc)2
PdCl2
glycerol
glycerol
94
53
(71)c
6.0:4.0
4.5:5.5
(6.2:3.8)c
7.5:2.5
5.0:5.0
-
55
24
(44)c
31
48
-
-
9
PdBr2
Pd(TFA)2
PdCl2
-
-
-
-
glycerol
glycerol
PhMe
41
10
11
12
97
<3
Pd(OAc)2
PhMe
<3
-
-
13
Pd/C
-
glycerol
>99
(73)d
7.0:3.0
70
(51)d
(7.0:3.0)d
a Reactions carried out under the general experimental procedure (see ESI for details).
b
c
Determined by GC-MS after 18 h of reaction time unless otherwise specified. In
brackets, results at 120 h. d In brackets, results at 6 h.
With the aim of evidencing organic intermediates in the Pd-
catalysed homo-coupling process, we ran the experiments in
the presence of 2,6-di-tert-butyl-4-methylphenol (BHT) as a
radical trap (2 equiv.); unreacted substrate was recovered up
to 98%, being the homo-coupling product and the hydro-
dehalogenated by-product only detected at trace levels (less
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than 2%, determined by GC-MS), proving that radical Firstly, the formation of such radical species would have led to
intermediates are involved in the reaction pathway. In order to mixture of regioisomers; however, the product
a
evidence the formation of radical species, we carried out corresponding to the activation of the bromo position was
Electronic Paramagnetic Resonance (EPR) studies for our exclusively obtained. Secondly, we carried out the reaction in
benchmark reaction, 4′-bromoacetophenone, hydroquinone (1 the presence of a large excess (up to 10 equiv.) of different
equiv.) and KOH (1 equiv.) in glycerol using PdOAcNPs as arenes [namely, fluorobenzene, α,α,α-trifluorotoluene, and
catalyst (1 mol% total palladium). These studies revealed the 1,3-bis(trifluoromethyl)benzene] with the aim of trapping
formation of the anion radical of 4’-bromoacetophenone as short-lived acetophenone radicals through formation of cross-
reaction intermediate during the homo-coupling reaction coupling products by C-H activation (Scheme 1), but these
(species I
in Figure 4), together with the benzoquinone anion expected products were not observed.49, 50 Taking into account
radical (species II in Figure 4).46-48 Thus far, 4′- these results, we propose that the corresponding
bromoacetophenone radical anions could not be detected in acetophenone radicals are not formed (not detected by EPR,
the absence of hydroquinone (which may act as a radical see Fig. S6 in the ESI), and a base-promoted homolytic
stabiliser).
aromatic substitution mechanism can be ruled out (see Fig. S7
Thus, these results confirm the operating radical pathway for in the ESI).
the PdNPs catalysed homo-coupling reaction in glycerol.
COMe
MeOC
Br
+
Br-
MeOC
MeOC
MeOC
I
only 1,4-disubstituted
diphenyl observed
R1
R2
COMe
R1 = F; R2 = H
R1 = CF3; R2 = H
R1, R2 = CF3
MeOC
R1
+
MeOC
R2
not detected
only formed
Scheme 1. Ruled out pathways for the acetophenone radical anion (species I).
The presence of such radical species could not be explained
through a classical oxidative addition of bromoarenes on Pd(0)
to generate a molecular Pd(II) complex that could then
undergo a second oxidative addition through the formation of
highly reactive Pd(IV) species. Thus, we focused on the
hypothesis that the activation of bromoarenes could take
place via neighbouring effects engaging different metal
centres, thanks to the electronic structure of the metal
nanoparticles (through the electrons at the Fermi level).
These EPR studies, together with the halide effect on the metal
surface and the use of a radical trap enable us to propose a
radical catalytic cycle probably involving
a Pd(0)/Pd(I)
manifold. These results highlight the importance of
nanocatalysts to allow new reaction pathways different to
those of classical homogeneous or heterogeneous catalysts.
Overall, the combined experimental evidences enable us to
propose a novel reaction mechanism based on an activation of
aryl bromides onto the surface of the nanoparticles by
intermetallic cooperative effects.
Figure 4. EPR spectra: a) Experimental EPR spectrum of the reaction sample; b)
Superposition of simulated EPR spectra of both anion radical species of 4′-
bromoacetophenone (I) and hydroquinone (II); c) Simulated EPR spectrum of 4′-
bromoacetophenone anion radical (species I); d) Simulated EPR spectrum of
hydroquinone anion radical (species II).
Despite the fact that we did not observe the acetophenone
radical by EPR, we paid special attention to identify any
potential coupling product that could confirm its formation via
a base-promoted homolytic aromatic substitution mechanism.
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processes in comparison to classical catalytic systems due to
their defined morphology and composition, thereby impacting
on the operating catalytic regimes.
In addition, we have experimentally shown that the
nanocatalysed homo-coupling reaction proceeds via radical
anion intermediates. By exploiting the surface reactivity of
nanoparticles, it is feasible to invoke formal intermediate
oxidation states, and exploit these intrinsic properties towards
Pd-catalysed homo-couplings of aryl halides. For selected
substrates, high conversions were achieved, furnishing the
desired bis-aryl products in high yields. From a mechanistic
point of view, we hypothesized that the activation of aryl
halides via
a surface mechanism may be achieved via
neighbouring effect involving multiple metal centres, most
likely through the electrons at the Fermi level. The mechanism
of the coupling reaction was studied by means of radical trap
and electron paramagnetic resonance (EPR) experiments,
which supports
a radical-based pathway. In particular,
experimental evidence of the presence of aryl bromide radical
anions as intermediates in the homo-coupling reaction has
been provided, together with the confirmation that this
transformation occurs under a radical mechanism as the
homo-coupling reaction is precluded when BHT is employed as
a radical scavenger.
These results highlight the importance of nanocatalysts with
enhanced active surface area towards novel reaction
manifolds that were otherwise limited to classical
heterogeneous systems.
Scheme 2. Plausible reaction mechanism for the reductive homocoupling reaction
mediated by Pd nanoparticles.
Conflicts of interest
Conclusions
There are no conflicts to declare.
This fundamental study sheds light on how the nature of
interactions at the metal surface of Pd(0) NPs plays a crucial
role in determining the reactivity of catalytic systems,
ultimately fostering a reaction manifold, or shutting down a
reaction pathway. Notably, the discriminating role of the
additives arising from the precursor salts and their impact
towards gaining control of the chemoselectivity has been
evidenced for catalytic transformations mediated by metallic
nanoparticles in glycerol. In particular, we have identified the
crucial role of counteranions on tuning the reactivity of the
described catalytic systems. Thus, the presence of iodide
anions serves as an effective poisoning additive to switch off
the reductive homocoupling reaction manifold, probably due
to both their strong interaction with the palladium surface and
their low solvation by glycerol.
Acknowledgements
The Centre National de la Recherche Scientifique (CNRS), the
Université de Toulouse 3 – Paul Sabatier and Chemium SPRL
are gratefully acknowledged for their financial support. The
authors thank T. Girard, M. Ceresiat and L. Reginster from
Chemium SPRL for fruitful discussions. The authors
acknowledge C. Pradel for performing the TEM analyses, R.
Lenk and E. Bellan for their help concerning the EPR studies,
and N. López and M. Ortuño for mechanistic discussions.
Notes and references
Herein, the importance of the surface reactivity for homo-
coupling transformations could be highlighted in the present
study through comparison to catalyst controls (namely Pd/C
and CataCXium® C, respectively). Despite the difficulty to work
under pure heterogeneous or homogenous regimes, as both
leaching of molecular species in the former, or intermetallic
bond formation leading to clusters and agglomeration in the
latter can occur, the tailored nanoparticles prepared in this
study, and PdOAcNPs in particular, provide a direct entry
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DOI: 10.1039/C8CY00901E
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