Tuning and enhancement of the Mizoroki-Heck reaction using polarized Pd nanocomposite carbon aerogels

Article abstract of DOI:10.1039/c6nj02279k

The application of a positive electric field on a Pd(0)-carbon aerogel nanocomposite enhances the reaction rate of the Mizoroki-Heck reaction. By contrast, when a negative potential is employed the reaction is inhibited. On the other hand, chronoamperometry measurements have revealed the polarizing current intensity as an indirect measure of the reaction evolution. The cationic nature of the reaction intermediates was unambiguously confirmed and a polar mechanism has been finally proposed.

Full text of DOI:10.1039/c6nj02279k

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Vallribera, E.
Molins and L. Martín, New J. Chem., 2016, DOI: 10.1039/C6NJ02279K.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/njc

Please do not adjust margins
New Journal of Chemistry
Page 1 of 5
View Article Online
DOI: 10.1039/C6NJ02279K
Journal Name
COMMUNICATION
Tuning and enhancement of the Mizoroki-Heck reaction using
polarized Pd nanocomposite carbon aerogels
Laura Martín,a,b Elies Molins,*,b and Adelina Vallribera,*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The application of a positive electric field on a Pd(0)-carbon field manipulate a M-H reaction? Taking advantage that
aerogel nanocomposite enhances the reaction rate of the Pd(0)Nps can be fixed on a high surface area carbon aerogel,
Mizoroki-Heck reaction. By contrast, when a negative potential is which is highly porous and electrically conductive (25-100 S
employed the reaction is inhibited. On the other hand, cm-1),7 we envisaged to apply the original idea of Vayenas in
chronoamperometry measurements have revealed the polarizing the use of Pd(0)-carbon aerogels nanocomposites (Pd(0)-CAs)
current intensity as an indirect measure of the reaction evolution. as electrode and catalyst at the same time. As M-H reaction is
The cationic nature of the reaction intermediates was mediated on transition metal atoms in which oxidative
unambiguously confirmed and a polar mechanism has been finally addition, alkene insertion and reductive elimination occur,8 we
proposed.
are not at all contemplating a redox electrocatalytic process
between Pd to Pd2+. In fact, the electrochemical oxidation of
palladium is not possible at the electric potentials applied
along in this work.9
In 2002, Vayenas et al.1 described that the rate of many
heterogeneous chemical reactions can be modified by altering
the potential between the catalyst and a reference electrode.
Related to this concept, recently theoretical calculations
suggest that external electric fields (EEFs) can affect the results
of several chemical reactions even if no metals are involved. It
has been theoretically demonstrated that EEFs can control the
selectivity of enzymatic-like bond activations,2 as well as
chemical reactions that involve bond forming and bond-
breaking steps.3 Moreover, the ability to control reaction rates
with an applied electrical potential gradient has been recently
demonstrated for a Diels-Alder reaction using the scanning
tunneling microscopy break-junction approach to deliver an
electric-field stimulus.4
Metal-doped CAs are well-known5c,6,10-11 since the pioneering
work of Baumann et al.11b who used sol-gel polymerization of
resorcinol derivatives containing ion-exchange moieties,
allowing the incorporation of the metal ions uniformly into the
aerogel matrix. In this work Pd(0) nanocomposited CAs were
synthesized following this methodology.11b Once the organic
gel was obtained, a supercritical drying process was performed
and a subsequent pyrolysis step gave rise to the Pd(0)-CA. The
Pd(0)-CAs presented high surface area (401 m2g-1), low bulk
density (0.65 gcm-3) and high metal content (39 wt % of Pd).
The corresponding powder X-ray diffraction (XRD) patterns
were characteristic of a metallic fcc-Pd phase (Fig. 1-B) and no
presence of PdO was detected. Transmission electron
microscopy (TEM, Fig. 1-C1) images showed spherical
nanoparticles with a mean particle size of 17 ± 7 nm (Fig. 1-
C2).
In our own research, we have extensively used Pd(0)Nps
embedded in silica and carbon aerogels5 as recoverable
catalysis for cross-coupling reactions, among them the
Mizoroki-Heck (M-H).6 In continuation of this research, the aim
of this work is to answer this question: Can an external electric
The electrode-catalyst was obtained through the incorporation
of a graphite rod (PGR) into a precursory gel as current
collector (Fig. 1-A2). Prior to the drying process and further
pyrolysis, the graphite rods were filed off and treated with
KOH, thus improving the interaction between its surface and
the organic matrix of the gel.12 Commercial pencil leads (CPLs)
were also used as current collectors. Cyclic voltammetry
measurements comparing the different working electrodes
were carried out. The results showed that Pd(0)-CA with PGR
presented higher current density values.
a.Department of Chemistry and Centro de Innovación en Química Avanzada
(ORFEO-CINQA). Universitat Autònoma de Barcelona. Campus UAB, 08193
Bellaterra, Spain. E-mail: adelina.vallribera@uab.cat
b.Department of Crystallography. Institut de Ciència de Materials de Barcelona
(ICMAB-CSIC). Campus UAB, 08193 Bellaterra, Spain
Electronic Supplementary Information (ESI) available: synthesis of Pd(0)-CA, cyclic
voltammetry and chronoamperometry measurements; procedure for M-H
reactions under electric field; TEM, SEM, XRD and EDX characterization of
materials; graphics for current intensity evolution; characterization of 5-7
.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 2 of 5
COMMUNICATION
Journal Name
advantage of our method is that the use of thisViPewdArrteiccleoOvnelirnye
process leads to an excellent recover ofDpOaIl:la10d.i1u0m39./CC6oNmJ0p2a2r7e9dK
to our previous published results (leaching about 3 wt% of Pd)
for the same catalytic reaction,6a analysis of the crude
corresponding to a +0.5 V reaction mixture after that Pd
recovery process indicated a low leaching of 0.2 wt% of the
palladium present in the Pd(0)-CA. Then, consecutive reactions
under positive and negative potentials were carried out using
the same batch of Pd(0)CA. In this conditions Pd(0)-CA was
reused 5 times with excellent results (99, 98, 98, 98, 92 % yield
at 3h reaction time), although a decrease in activity was
observed from the 6th cycle (see Table S2 in ESI for the results
of 9 cycles). TEM images of Pd(0)-CA after 9 cycles showed that
Fig. 1. (A1): Photograph image of a piece of Pd(0)-CA; (A2): Photograph palladium was recovered in the fcc-Pd(0) nanoparticle form
image of Pd(0)-CA incorporated in a graphite rod; (A3) Schematic with a mean particle size of 16 nm. Hence, these results
representation of the Pd(0)-CA structure with Pd(0)Np into the CA confirm the recyclability of the Pd(0)-CA and that the Pd
matrix; (B) Powder XRD pattern; (C1) TEM image; (C2) Particle size recovery process preserve the initial characteristics of the
distribution.
palladium supported on CA.
The M-H reaction selected was the reaction of iodobenzene, 1,
with ethyl acrylate,
4 (scheme 1). The potentials tested were
+1, +0.5, -0.5 and -1 V (vs Pt), as shown in Table 1 (entries 1, 2,
5, 6). Unconnected electrodes and 0.0 V reactions were used
as comparison references, using the same molar percentage of
Pd as the corresponding reaction (entries 3, 4). According to Scheme 1. M-H reactions
the results (entries 1-6), positive potentials gave higher Table 1. Results and conditions of the M-H reactions (scheme 1)[a]
conversions (89-94%), whereas negative potentials gave lower
ones (45-46%), relative to systems with unconnected
electrodes and 0.0 V reactions (55 and 66% yield respectively).
Thus, the reaction is further activated when the potential is
positive and inhibited when it is negative. According to these
experimental results, our hypothesis is that a change in the
Fermi level of the PdNp occurs when an EEF is applied over the
Pd(0)-CA. This behavior allows the tuning and enhancing of the
reaction rate when positive potential is applied. An increase in
the reaction rate was also observed by Tian and Moeller when
applying constant current during the M-H reaction instead of a
potential. However, no difference in the reaction rate using
negative or positive current was observed.13 To ensure a
minimum current necessary for an EEF stimulus, preliminary
M-H experiments were carried out with the addition of
tetrabutylammonium tetrafluoroborate (TBABF4) as electrolyte
We further examined the reaction in the absence of
electrolyte and, surprisingly, almost the same behavior was
observed (Table 1, compare entries 13 and 14 with entries 1
and 2 and see ESI (Tables S1 and S3). Hence, all the
experiments were then performed without electrolyte. With
Entry
Aryl
halide
Pd(0)-CA
mol %
E(V) vs Pt
Yields
% (3 h)
89
94
56
66
45
46
97
27
94
0
99
80
99
92
1
2
3
4
5
6
7
8
9
10
11
12
13[c]
14[c]
1
1
1
1
1
1
1
1
2
2
3
3
1
1
7
7
7
7
7
+1
+0.5
0.0
unconnected
-0.5
7
-1
+0.5
0.0
+0.5
0.0
+0.5
0.0
4[b]
4[b]
5
5
8
8
7
+0.5
+1
7
[a]Conditions: aryl iodide (6 mmol), ethyl acrylate (18 mmol), Et3N (12
mmol) at 90 ºC in CH3CN (25 ml), without using electrolyte, and
applying the potential indicated in each case. The working electrode
[b]
was a Pd(0)-CA with CPL as current collector. Using Pd(0)-CA with
PGR as current collector.[c] TBABF4 (0.1 M) was employed as
electrolyte.
other tested aryl iodides (2 and 3), an enhancement of the
reaction was also achieved using +0.5 V, compared to the
results obtained with 0.0 V (Table 1; entries 9 to 12).
According to the results discussed so far, activation of
palladium is achieved when it is present in any positively
charged electrode. We have observed that palladium is always
transported to the cathode, from anywhere deposited, when
potential is applied (either positive or negative). Palladium
anode deposition has never been observed during the
reaction. All of this suggests that cationic palladium
intermediates must be generated during the M-H reaction
cycle.
In the reactions using positive potential and when conversion
was close to 100%, deposition of black palladium on the Pt
counter electrode Pt(C) and throughout the cell was observed
by SEM and EDX analysis (see ESI). We discovered that the
application of -1 V once the reaction was over, made the Pd
deposited came back to the Pd(0)-CA. It is accepted that Pd
cross-coupling reactions are in general homogeneous
processes, even if heterogeneous catalysts are used,5-6 and the
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 5
New Journal of Chemistry
Please do not adjust margins
Journal Name
COMMUNICATION
Chronoamperometry measurements of the reaction (Table 1, observed palladium transport exclusively to the cathode,
View Article Online
DOI: 10.1039/C6NJ02279K
entries 7 and 8) have been carried out at +0.5 V (orange plot, which can be explained if cationic palladium complexes are
Fig. 2A) and 0.0 V (blue plot, Fig. 2A). At the beginning of the generated during the cycle. Besides the cathode, Pd(II) can be
reaction, applying +0.5 V, there were very low current values reduced to Pd(0) by the action of the Et3N and this Pd(0) is
(between 10-20 microampere when just the potential is deposited on the Pt(C) (cathode). The deposition of palladium
applied), but they increased up to a maximum (point A) in throughout the cell could be due to the reduction of Pd(II) by
parallel with the evolution of the reaction. Then, the current Et3N during its transport to the cathode, and consequent
slightly decreases and finally stabilizes when the reaction is aggregation and precipitation. As mentioned, applying -1 V
almost complete. So, the current appears as an indirect once the reaction is over, the Pd deposited on Pt(C) and in the
measure of the reaction evolution. With 0.0 V, the reaction is cell can be recovered again in the aerogel.
not enhanced and proceeds slowly with very small current
intensities detected. The increase of the current observed in
5.5 h - 100% conv.
A
6
4
2
0
3 h - 97% conv.
●
A
+0.5 V vs. Pt
the +0.5
V reaction should be due to the increasing
0,2
0,0
concentration of ionic species that must be generated during
the M-H reaction, since at the beginning of the reaction the
current values are very low. These species could be the Et3NH+
3 h - 27% conv.
0.0 V vs. Pt
-0,2
-0,4
5.5 h - 79% conv.
24 h - 100% conv.
15 20 25
I- generated at every catalytic cycle and other possible ionic
0
5
10
time (h)
8a,d
intermediates.14,
The decrease of the current after the
3 h - 27% conv.
5.5 h - 79% conv.
0.0 V vs. Pt
6
7
maximum (point A, Fig. 2A) indicates a decrease of the
concentration of ionic intermediates since the reaction is
ending, and then, only Et3NH+ I- remains with an almost
constant concentration, which should be responsible of the
current stabilization. In Fig. 2B the reaction behavior applying
+0.5 V and -0.5 V using electrolyte is represented. When
electrolyte is added, higher current values are observed when
a potential is applied. After a complete reaction at +0.5 V, we
applied -0.5 V and the Pd deposited on the Pt(C), acting as the
anode, led also to the activation of the reaction rate.
Moreover, both reactions have the same behavior (Fig. 2B).
0
1
2
3
4
5
time (h)
B
8
4
3 h - 100% conv.
+0.5 V vs. Pt
0
3 h - 100% conv.
-0.5 V vs. Pt
-4
0
1
2
3
time (h)
(conv. = conversion of final product)
These results gives further evidence that, when Pd is deposited Fig. 2. Chronoamperometry measurements for the M-H reaction of
at the anode, negative and positive potentials achieved and (A) The working electrode was a Pd(0)-CA with PGR as current
activation of Pd and the current appears as an indirect collector. The gap after each time control (represented by dots)
measure of the reaction evolution. corresponds to the time sampling. The reactions were done applying
Taking into account all the results obtained and those constant potentials of +0.5 V and 0.0 V (vs Pt). (B) Reaction with
1
4
8b, 15-17
previously published14,
we propose a mechanistic electrolyte (TBABF4 0.1 M) and applying constant potentials of +0.5 V
hypothesis for the EEF accelerated M-H reaction at positive and -0.5 V (vs Pt). The working electrode was a Pd-CA(0) with CPL as
potential (Fig. 3). Enhancement of the Pd reactivity by positive current collector.
potential favors the oxidative reaction of the supported Pd(0) The sigmoidal profile of the current vs. time (Fig. 2A, orange
with the aryl halide. The reaction occurs on the surface of the curve) can be understood in the Michaelis-Menten kinetics
Pd(0) nanoparticles supported on the CA. Once the reaction framework as the addition of the contributions of the two
has started small amounts of palladium are dissolved in the main cationic carriers (Et3NH+ as one final product and the
form of Pd(II) soluble complexes (ArPd(CH3CN)(Et3N)I) palladium intermediates complexes [XPd(CH3CN)2(Et3N)]+) of
produced from the oxidative addition of the aryl iodide. In our the M-H reaction.
conditions, CH3CN and Et3N coordinate to the metal as We then decided to investigate the M-H reaction with
stabilizing ligands.15 The equilibrium of iodide dissociation alternating current. This process is carried out alternating
leads to the formation of
a
cationic complex positive and negative potentials in regular periods of time
([ArPd(CH3CN)(Et3N)]+).14,17 Ar-Pd+ intermediates are metal- during the course of the reaction, so the positive potential
centered electrophiles attacking the double bond of the ethyl activates the reaction and the negative potential inverts the
acrylate in a sort of electrophilic addition process. The palladium deposition on the Pt(C), also promoting the M-H
migratory insertion is the product-forming step of the M-H reaction (see ESI). We have performed only some preliminary
cycle, in which a C-C bond is formed. Then palladium hydride is essays although they confirm the mechanisms proposed. This
eliminated ([HPd(CH3CN)(Et3N)]+) to release the double bound process must be studied in more detail to find out the optimal
and finally the Pd(0) is released and launches the next cycle. conditions just reaching high reaction rates and avoiding
However, when the concentration of aryl halide is low enough, palladium deposition.
that is when the reaction is finishing, launches of Pd(0) is In conclusion, we have demonstrated enhancement of the M-
slower and transport of palladium to the cathode may occur in H reaction using Pd(0)-CA when a positive electric field is applied.
the form of a cationic hydride complex. This agrees with the The opposite is observed when using a negative field. Thus, the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 4 of 5
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C6NJ02279K
Fig. 3. Proposed mechanism for the enhanced M-H reaction with E>0. A TEM image has been used to represent the Pd(0)-CA electrocatalyst. W:
working electrode; C: counter electrode; Ref: reference electrode.
3
4
R. Meir, H. Chen, W. Lai and S. Shaik, ChemPhysChem, 2010
11, 301.
A. C. Aragonès, N. L. Haworth, N. Darwish, S. Ciampi, N. J.
Bloomfield, G. G. Wallace, I. Diez-Perez and M. L. Cote,
Nature, 2016, 531, 88.
a) S. Cacchi, C. L. Cotet, G. Fabrizi, G. Forte, A. Goggiamani, L.
Martín, S. Martínez, E. Molins, M. Moreno-Mañas, F.
Petrucci, A. Roig and A. Vallribera, Tetrahedron, 2007, 63
2519; b) R. Soler, S. Cacchi, G. Fabrizi, G. Forte, L. Martín, S.
Martínez, E. Molins, M. Moreno-Mañas, F. Petrucci, R. M.
Sebastián and A. Vallribera, Synthesis, 2007, 3068. c) L.
Martín, E. Molins and A. Vallribera, Tetrahedron, 2012, 68
6517.
a) S. Martínez, A. Vallribera, C. L. Cotet, M. Popovici, L.
Martín, A. Roig, M. Moreno-Mañas and E. Molins, New J.
Chem., 2005, 29, 1342; b) S. Martínez, M. Moreno-Mañas, A.
Vallribera, U. Schubert, A. Roig and E. Molins, New J. Chem.,
2006, 30, 1093.
J. Li, X. Wang, Q. Huang, S. Gamboa and P. J. Sebastian, J.
Power Sources, 2006, 158, 784.
,
effect of the electric field on the PdNps appears as tuning the
catalytic activity of Pd. This ability to manipulate chemical
reactions with electric fields offers proof-of-principle for other
heterogeneous catalysts. Moreover, the current intensity
appears as an indirect measure of the reaction evolution,
which is strongly related with the concentration of reaction
intermediates. This method, which probably may be extended
to other synthetic processes, allows analyzing the kinetics of
the reaction, being in the current case of the Michaelis-
Menten type. Finally, a mechanistic hypothesis based on
cationic palladium intermediates and according to the results
observed is proposed.
5
6
,
,
Experimental Section
General procedure for Pd(0)-CA catalyzed M-H reaction under
electric stimulus. A three-electrode system composed of Pd(0)-
CA as working electrode, a platinum cylinder (9 cm2) as
counter electrode and a platinum wire (0.6 mm of diameter) as
quasireference electrode were used. The reactions were
carried out under reflux (90 ºC), in the presence of air,
magnetic stirring and constant potential. Ethyl acrylate (18
mmol), aryl iodide (6 mmol) and triethylamine (12 mmol) were
dissolved in 25 mL of acetonitrile. When the reaction was over,
the solution was decanted and the Pd(0)-CA washed several
times with acetonitrile. The combined solvent extracts were
evaporated, the residue was dissolved in CH2Cl2 and the
solution was washed with aqueous NaHCO3 and a saturated
solution of NaCl. The organic extract was evaporated to afford
the coupling product. The Pd(0)-CA was always submerged in
CH3CN 24 h before its use and kept in the same solvent.
Acknowledgements
This research was supported from Spain MICINN (CTQ2011-
22649 and ENE2012-36368, ENE2015-63969) and MEC
(CTQ2014-53662-P and CTQ2014-51912-REDC) and the DURSI-
Generalitat de Catalunya (2014SGR-1643 and 2014SGR-2016).
L.M. was funded through a fellowship from the MICINN. We
acknowledge helpful discussions with Profs. N. Casañ and J.
Llorca.
7
8
a) I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000,
100, 3009; b) X.-F. Wu, P. Anbarasan, H. Neumann and M.
Beller, Angew. Chem. Int. Ed., 2010, 49, 9047; c) X.-F. Wu, P.
Anbarasan, H. Neumann and M. Beller, Angew. Chem. Int.
Ed., 2010, 49, 9047; d) J. P. Knowles and A. Whiting, Org.
Biomol. Chem., 2007, 5, 31.
9
a) M. Grdeń, M. Łukaszewski, G. Jerkiewicz and A.
Czerwiński, Electrochimica Acta, 2008, 53, 7583; b) A. J. Bard
and L. R. Faulkner, Electrochemical Methods. Fundamentals
and Applications, 2nd edn, John Wiley and Sons Inc., 2001.
10 A. Vallribera and E. Molins, Aerogel Supported Nanoparticles
in Catalysis in Nanoparticles and Catalysis, Ed. D. Astruc,
Wiley-VCH, 2008, 161-194.
11 a) C. Moreno-Castilla and F. J. Maldonado-Hódar, Carbon,
2005, 43, 455; b) T. F. Baumann, G. A. Fox, J. H. Satcher, Jr.,
N. Yoshizawa, R. Fu and M. S. Dresselhaus, Langmuir, 2002,
18, 7073; b) F. J. Maldonado-Hódar, C. Moreno-Castilla and
A. F. Pérez-Cadenas, Microporous Mesoporous Mater., 2004,
69, 119; c) L.C. Cotet, M. Gich, A. Roig, I. C. Popescu, V.
Cosoveanu, E. Molins and V. Danciu, J. Non-Cryst. Solids,
2006, 352, 2772.
12 S. Park and J. Kim, J. Colloid Interface Sci., 2000, 232, 311.
13 J. Tian and K. D. Moeller, Org. Lett., 2005,
14 A. Jutand, Chem. Rev., 2008, 108, 2300.
15 F. Zhao, M. Shirai, Y. Ikushima and M. Arai, J. Mol. Catal. A:
Chem., 2002, 180, 211.
16 A. Biffis, M. Zecca and M. Basato, Eur. J. Inorg. Chem., 2011,
1131.
7, 5381.
Notes and references
1
C. G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda and D.
Tsiplakides, Electrochemical Activation of Catalysis, Kluwer
Academic, 2002.
S. Shaik, S. P. de Visser and D. Kumar, J. Am Chem Soc., 2004
126, 11746.
17 C. Amatore, B. Godin, A. Jutand and F. Lemaître, Chem. Eur.
J., 2007, 13, 2002.
2
,
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
New Journal of Chemistry
View Article Online
DOI: 10.1039/C6NJ02279K