Thermal and electrochemically assisted Pd-Cl bond cleavage in the d 9-d9 Pd2(dppm)2Cl2 complex by Pd3(dppm)3(CO)n+ clusters (n = 2, 1, 0)

Article abstract of DOI:10.1021/ic061777h

A new aspect of reactivity of the cluster [Pd₃(dppm) ₃(μ³-CO)]ⁿ⁺, ([Pd₃]ⁿ⁺, n = 2, 1, 0) with the low-valent metal-metal-bonded Pd₂(dppm) ₂Cl₂ dimer (Pd₂Cl₂) was observed using electrochemical techniques. The direct reaction between [Pd ₃]²⁺ and Pd₂Cl₂ in THF at room temperature leads to the known [Pd₃(dppm)₃(μ ³-CO)(Cl)]⁺ ([Pd₃(Cl)]⁺) adduct and the monocationic species Pd₂(dppm)₂Cl⁺ (very likely as Pd₂(dppm)₂(Cl)(THF)⁺, [Pd ₂Cl]⁺) as unambiguously demonstrated by UV-vis and ³¹P NMR spectroscopy. In this case, [Pd₃]²⁺ acts as a strong Lewis acid toward the labile Cl^- ion, which weakly dissociates from Pd₂Cl₂ (i.e., dissociative mechanism). Host-guest interactions between [Pd₃]²⁺ and Pd ₂Cl₂ seem unlikely on the basis of computer modeling because of the strong screening of the Pd-Cl fragment by the Ph-dppm groups in Pd₂Cl₂. The electrogenerated clusters [Pd ₃]⁺ and [Pd₃]⁰ also react with Pd₂Cl₂ to unexpectedly form the same oxidized adduct, [Pd₃(Cl)]⁺, despite the known very low affinity of [Pd₃]⁺ and [Pd₃]⁰ toward Cl ^- ions. The reduced biproduct in this case is the highly reactive zerovalent species "Pd₂(dppm)₂" or "Pd(dppm)" as demonstrated by quenching with CDCl₃ (forming the well-known complex Pd(dppm)Cl₂) or in presence of dppm (forming the known Pd₂(dppm)₃ d¹⁰-d¹⁰ dimer). To bring these halide-electron exchange reactions to completion for [Pd ₃]⁺ and [Pd₃]⁰, 0.5 and 1.0 equiv of Pd₂Cl₂ are necessary, respectively, accounting perfectly for the number of exchanged electrons. The presence of a partial dissociation of Pd₂Cl₂ into the Cl^- ion and the monocation [Pd₂Cl]⁺, which is easier to reduce than Pd ₂Cl₂, is suggested to explain the overall electrochemical results. It is possible to regulate the nature of the species formed from Pd₂Cl₂ by changing the state of charge of the title cluster.

Full text of DOI:10.1021/ic061777h

Inorg. Chem. 2007, 46, 3083−3088
Thermal and Electrochemically Assisted Pd
−
Cl Bond Cleavage in the
+
d9
(n
−
)
d9 Pd2(dppm)2Cl2 Complex by Pd3(dppm)3(CO)n Clusters
2, 1, 0)
Cyril Cugnet,1a Yves Mugnier,*,1a Sophie Dal Molin,1a David Brevet,1a Dominique Lucas,1a and
Pierre D. Harvey*,1b
Institut de Chimie Mole´culaire de l’UniVersite´ de Bourgogne, (ICMUB) UMR CNRS 5260,
UniVersite´ de Bourgogne, Faculte´ des Sciences Mirande, 9 Alle´e A. SaVary, 21000 Dijon, France,
and De´partement de Chimie, UniVersite´ de Sherbrooke, Sherbrooke, Que´bec, Canada J1K 2R1
Received September 19, 2006
A new aspect of reactivity of the cluster [Pd3(dppm)3(µ ) 2, 1, 0) with the low-valent metal−
3-CO)]n+, ([Pd3]n+, n
metal-bonded Pd2(dppm)2Cl2 dimer (Pd2Cl2) was observed using electrochemical techniques. The direct reaction
+
between [Pd3]2 and Pd2Cl2 in THF at room temperature leads to the known [Pd3(dppm)3(
µ
3-CO)(Cl)]+ ([Pd3(Cl)]+)
adduct and the monocationic species Pd2(dppm)2Cl+ (very likely as Pd2(dppm)2(Cl)(THF)+, [Pd2Cl]+) as unambiguously
+
demonstrated by UV
−
vis and 31P NMR spectroscopy. In this case, [Pd3]2 acts as a strong Lewis acid toward the
labile Cl- ion, which weakly dissociates from Pd2Cl2 (i.e., dissociative mechanism). Host
−guest interactions between
[Pd3]2 and Pd2Cl2 seem unlikely on the basis of computer modeling because of the strong screening of the Pd
−Cl
+
fragment by the Ph-dppm groups in Pd2Cl2. The electrogenerated clusters [Pd3]+ and [Pd3]0 also react with Pd2Cl2
to unexpectedly form the same oxidized adduct, [Pd3(Cl)]+, despite the known very low affinity of [Pd3]+ and [Pd3]0
toward Cl- ions. The reduced biproduct in this case is the highly reactive zerovalent species “Pd2(dppm)2” or
“Pd(dppm)” as demonstrated by quenching with CDCl3 (forming the well-known complex Pd(dppm)Cl2) or in presence
of dppm (forming the known Pd2(dppm)3 d10 d10 dimer). To bring these halide-electron exchange reactions to
−
completion for [Pd3]+ and [Pd3]0, 0.5 and 1.0 equiv of Pd2Cl2 are necessary, respectively, accounting perfectly for
the number of exchanged electrons. The presence of a partial dissociation of Pd2Cl2 into the Cl- ion and the
monocation [Pd2Cl]+, which is easier to reduce than Pd2Cl2, is suggested to explain the overall electrochemical
results. It is possible to regulate the nature of the species formed from Pd2Cl2 by changing the state of charge of
the title cluster.
Introduction
formation of acid fluorides, dissymmetric esters and ethers,
diradical couplings, and C-H bond activation).3 While these
investigations focused on organic substrates, the inorganic
counterpart (M-X) was neglected. The abstraction of a Cl-
ligand of M-X (X ) Cl) by the [Pd3(dppm)3(µ3-CO)]2+
cluster to form the known adduct [Pd3(dppm)3(µ3-CO)(Cl)]+
([Pd3(Cl)]+) yielding the monocation derivative M+ repre-
sents an interesting avenue for catalysis because the Cl- ions
Homogeneous catalytic processes using transition metal
complexes often require a key step of activation of the so-
called precatalyst. This primary step may be an abstraction
of a blocking halide ligand or an electron-transfer generating
what is often referred to as the active catalyst. Recently, our
groups reported an exhaustive series of reactivity and
properties of the title clusters (Chart 1),2 including host-
guest behavior,2d electrocatalytically induced C-X bond
activation (X ) Cl, Br, I), and electrocatalysis (for the
(2) (a) Cugnet, C.; Lucas, D.; Lemaˆıtre, F.; Collange, E.; Soldera, A.;
Mugnier, Y.; Harvey, P. D. Chem.sEur. J 2006, 12, 8386-8395. (b)
Brevet, D.; Lucas, D.; Richard, P.; Vallat, A.; Mugnier, Y.; Harvey,
P. D. Can. J. Chem. 2006, 84, 243. (c) Brevet, D.; Lucas, D.; Mugnier,
Y.; Harvey, P. D. J. Cluster Sci. 2006, 17, 5. (d) Harvey, P. D.;
Mugnier, Y.; Lucas, D.; Evrard, D.; Lemaˆıtre, F.; Vallat, A. J. Cluster
Sci. 2004, 15, 63 and references therein. (e) Lemaitre, F.; Lucas, D.;
Brevet, D.; Vallat, A.; Harvey, P. D.; Mugnier, Y. Inorg. Chem. 2002,
41, 2368.
* To whom correspondence should be addressed. Phone/Fax: 33 (0)3
80 39 60 91 (Y.M.). E-mail: yves.mugnier@u-bourgogne.fr (Y.M.);
pierre.harvey@usherbrooke.ca (P.D.H.). Phone: (819) 821-7092 (P.D.H.).
Fax: (819) 821-8017 (P.D.H.).
(1) (a) Universite´ de Bourgogne. (b) Universite´ de Sherbrooke.
10.1021/ic061777h CCC: $37.00
Published on Web 03/20/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 8, 2007 3083

Cugnet et al.
Chart 1
can be subsequently removed in a catalytic fashion.3b So,
this cluster can act as a cocatalyst.
We now wish to report the reactivity of [Pd3(dppm)3(µ3-
CO)]n+, ([Pd3]n+, n ) 2, 1, 0) toward Pd2(dppm)2Cl2 (Pd2-
Cl2)(Chart 1).
Figure 1. (a) RDE voltammograms of [Pd3]2+ (scan rate ) 0.02 V s-1
)
in a THF/Bu4NPF6 solution at room temperature and (b) after the addition
of 1 equiv of Pd2Cl2. Starting potential: 0 V.
Experimental Section
Materials. [Pd3(dppm)3(µ3-CO)](PF6)2,4 Pd2(dppm)2Cl2,5 and Pd-
6
(dppm)Cl2 were prepared according to literature procedures. All
solvents were dried under argon, and all experiments were
performed under argon.
Electrochemical Experiments. All manipulations were per-
formed using Schlenk techniques in an atmosphere of dry oxygen-
free argon gas. The supporting electrolyte was degassed under
vacuum before use and then solubilized at a concentration of 0.2
mol L-1. For cyclic voltammetry experiments, the concentration
of the analyte was approximatively 10-3 mol L-1. Voltammetric
analyses were carried out in a standard three-electrode cell with an
EG & G Princeton Applied Research (PAR) Model 263A
potentiostat connected to an interfaced computer employing
Electrochemistry Power Suite software. The reference electrode
was a saturated calomel electrode (SCE) separated from the solution
by a sintered glass disk. The auxiliary electrode was a platinum
wire. For all voltammetric measurements, the working electrode
was a vitreous carbon electrode (Φ ) 3 mm), and the angular
velocity was 26.5 rad s-1. In these conditions, when operated in
THF, the formal potential for the ferrocene ( couple is found to
be +0.56 V versus SCE. The controlled potential electrolysis
was performed with an Amel 552 potentiostat coupled with an
Amel 721 electronic integrator. High-scale electrolyses were
performed in a cell with three compartments separated with fritted
glasses of medium porosity. Carbon gauze was used as the working
electrode; a platinum plate was emplyed as the counter-electrode,
and a saturated calomel electrode was used as the reference
electrode.
Figure 2. (a) Cyclic voltammograms of [Pd3]2+ (scan rate ) 0.1 V s-1
in a THF/Bu4NPF6 solution at room temperature and (b and c) after the
addition of 1 equiv of Pd2Cl2. Starting potential: 0 V. Inversion potential:
(a and b) -2.5, (c) -1 V.
)
immediately, and an AgCl precipitate was formed. RMN 31P{1H}
(600 MHz, acetone-d6, 20 °C): δ 2.05 (t), -5.3 (t, J ) 43.5 Hz).
(+) MALDI-TOF MS: m/z (assignment, relative intensity) 982
(Pd2(dppm)2, 100%), 995 (Pd2(dppm)2O, 18.5%), 1017 (Pd2(dppm)2-
Cl, 6.5%), 1052 (Pd2(dppm)2Cl2 residue).
[Pd2(dppm)2(Cl)(MeCN)]+. One equivalent of AgPF6 (2.4 mg,
9.5µmol) was added to a solution of Pd2(dppm)2Cl2 (10 mg,
9.5.µmol) in MeCN (5 mL). The orange solution turned yellow
immediately, and an AgCl precipitate was formed. RMN 31P{1H}
(600 MHz, acetone-d6, 20 °C): δ 1.6 (t), -5.5 (t, J ) 42.5 Hz).
(+) MALDI-TOF MS: m/z (assignment, relative intensity) 982
(Pd2(dppm)2, 100%), 995 (Pd2(dppm)2O, 8.3%), 1017 (Pd2(dppm)2-
Cl, 6.7%), 1052 (Pd2(dppm)2Cl2 residue).
Spectroscopy. The 31P NMR spectra were recorded on a 600
MHz BRUKER Avance II NMR spectrometer. The chemical shifts
are reported with respect to H3PO4. Mass spectra were obtained
on a Bruker Daltonics Ultraflex II spectrometer (MALDI-TOF)
using dithranol as a matrix.
[Pd2(dppm)2(Cl)(THF)]+. One equivalent of AgPF6 (2.4 mg,
9.5 µmol) was added to a solution of Pd2(dppm)2Cl2 (10 mg,
9.5 µmol) in THF (5 mL). The orange solution turned dark orange
Modeling. The computer modeling was performed using the
commercial software PC-Model from Serena Software. It uses a
MMX force field for all atoms.
Results and Discussion
(3) (a) Lucas, D.; Lemaˆıtre, F.; Gallego-Gomez, B.; Cugnet, C.; Richard,
P.; Mugnier, Y.; Harvey, P. D. Eur. J. Inorg. Chem. 2005, 1011. (b)
Lemaˆıtre, F.; Lucas, D.; Groison, K.; Richard, P.; Mugnier, Y.; Harvey,
P. D. J. Am. Chem. Soc. 2003, 125, 5511. (c) Brevet, D.; Mugnier,
Y.; Lemaˆıtre, F.; Lucas, D.; Samreth, S.; Harvey, P. D. Inorg. Chem.
2003, 42, 4910. (d) Lemaˆıtre, F.; Lucas, D.; Mugnier, Y.; Harvey, P.
D. J. Org. Chem. 2002, 67, 7537. (e) Brevet, D.; Lucas, D.; Cattey,
H.; Lemaitre, F.; Mugnier, Y.; Harvey, P. D. J. Am. Chem. Soc. 2001,
123, 4340.
The voltammetric measurements, made using a rotating
disk electrode (RDE) for the [Pd3]2+ cluster in THF and in
the presence of 0.2 mol L-1 Bu4NPF6 as the supporting
electrolyte, exhibit two reduction waves A1 and A2 at -0.29
and -0.51 V versus SCE (Figure 1, trace a). The cyclic
voltammogram (CV) (Figure 2, trace a) exhibits the same
electrochemical processes as reversible systems (A1/A′1 and
(4) Manojlovic-Muir, L.; Muir, K. W.; Lloyd, B. R.; Puddephatt, R. J. J.
Chem. Soc., Chem. Commun. 1983, 1336.
(5) Benner, L. S.; Balch, A. L. J. Am. Chem. Soc. 1978, 100, 6099.
3084 Inorganic Chemistry, Vol. 46, No. 8, 2007

Pd-Cl Bond CleaWage
Chart 2
Coordination of a neutral ligand at the axial position of
the Pd2(dppm)22+ fragment is not uncommon. For example,
the disubstituted complex Pd2(dppm)2(NCCH3)22+ is known,9
along with the dissymmetric dimer Pd2(dppm)2(Cl)(NC5H4-
2-CHCH2)+.10
The 31P NMR spectra of the crude product also exhibit
two pseudo-triplets centered at 2.05 and -5.3 ppm; this is
best described as a AA′BB′ system that approximates a A2B2
spin system where the coupling constants are very similar
(J ) 43.5 Hz) as for the known dissymmetric dimers Pd2-
(dppm)2(Cl)(I) and Pd2(dppm)2(Br)(I) (J ) 39 Hz).11 In a
case where the J values are different, the spin system is best
described as AA′BB′ similar to that observed for dissym-
metric dimer Pd2(dppm)2(Br)(Cl) (J ) 39 Hz).11 From this
observed reactivity, one can readily conclude that [Pd3]2+ is
a stronger Lewis acid for Cl- than the unsaturated intermedi-
ate [Pd2(Cl)]+.
A2/A′2), which allows for the description of the following
redox reactions6
(A1/A′1)
[Pd3]2+ + e- y
z [Pd3]+
z [Pd3]0
(1)
(2)
(A2/A′2)
[Pd3]+ + e- y
In this way, cluster species [Pd3]+ and [Pd3]0 can be
electrochemically generated in the electrochemical cell prior
to the addition of Pd2Cl2.
Reactivity of [Pd3]2+ toward Pd2Cl2. In the presence of
one equivalent of Pd2Cl2, drastic modifications of the [Pd3]2+
RDE voltammogram are readily observed. The reduction
waves A1 and A2 disappear, and two well-defined reduction
waves N and A are observed. Wave A corresponds to the
reduction of Pd2Cl2.7 The current amplitude of wave N is
equal to the sum of heights of wave A1 and A2 (Figure 1).
A well defined absorption band is observed at 460 nm in
UV-vis spectra which corresponds to the adduct [Pd3-
(Cl)]+.3b,d,e In addition, after evaporation of the THF solvent,
the 31P NMR spectrum of the crude product exhibits several
signals, one of which is a sharp singlet at -9.8 ppm which
confirms the presence of [Pd3(Cl)]+; this value was also
obtained by addition of Cl- anion (Bu4NCl) to a solution
containing the title cluster.8 An independent NMR-tube
reaction of [Pd3]2+ with Pd2Cl2 monitored by 31P NMR gives
the same result as above.
The unexpected presence of wave A in trace b (Figure 2)
is caused by [Pd3(Cl)]+ being reduced to [Pd3]0 by a
2-electron process, which has no affinity for the Cl- ion.3b,e
So, these anions are released according to
[Pd3(Cl)]+ + 2e- f [Pd3]0 + Cl-
(4)
Then these released Cl- ions are captured by [Pd2Cl]+
according to eq 5, and Pd2Cl2 is regenerated
Pd2(dppm)2Cl(THF)+ + Cl- f Pd2(dppm)2Cl2 + THF (5)
Therefore, after scanning is completed through peak N,
derivatives [Pd3]0 and Pd2Cl2 are formed, and [Pd3]0 can be
oxidized back to [Pd3]2+ according to eqs 1 and 2 (oxidation
peaks A′1 and A′2, see Figure 2) during the anodic scan.
When the anodic sweep is started at -2.6 V, only oxidation
peak N′ is observed. This peak corresponds to the oxidation
of [Pd3]0 under the influence of Cl- (process ECE, electro-
chemical/chemical/electrochemical)2d which is formed by
reduction of Pd2Cl2 at the potential of peak A.
The presence of wave A in the resulting solution appears
odd at first glance because waves A1 and A2 disappear (i.e.,
[Pd3]2+ is totally consumed), indicating that a fast reactivity
between [Pd3]2+ and Pd2Cl2 occurs, whereas the reduction
wave A (reduction of Pd2Cl2) is still present, and no new
wave is observed attributable to the biproduct of [Pd3(Cl)]+.
In the CV, the oxidation peaks A′1 and A′2 are observed (i.e.,
[Pd3]0 is present) when the scan is reversed after peak N
(Figure 2, trace b) but are absent when the scan is reversed
after -2.6 V (i.e., [Pd3]0 is absent) and a well-defined
oxidation peak N′ is depicted.
We also considered a host-guest interaction, but this
hypothesis should readily be discarded because the Cl atom
in Pd2Cl2 is far too encumbered by the dppm-phenyl groups
where obviously strong steric interactions with [Pd3]2+ occur.
This feature is unambiguously demonstrated by computer
modeling (Figure 3). Instead, a dissociative mechanism must
be taken into account, as shown in eq 6
The presence of [Pd3(Cl)]+ indicates an heterolytic Pd-
Cl bond cleavage in Pd2Cl2 yielding [Pd2Cl]+ (Chart 2) in
which the THF ligand is most likely coordinated to the
palladium atom according to the following reaction
Pd2(dppm)2Cl2 + THF h Pd2(dppm)2Cl(THF)+ + Cl-
(6)
[Pd3]2+ + Pd2(dppm)2Cl2
8 [Pd3(Cl)]+ +
This proposal is entirely consistent with the relatively facile
ligand substitution reaction known for Pd2Cl2 and related
THF
Pd2(dppm)2Cl(THF)+ (3)
(6) Gauthron, I.; Mugnier, Y.; Hierso, K.; Harvey, P. D. Can. J. Chem.
1997, 75, 1182.
(7) Gauthron, I.; Mugnier, Y.; Hierso, K.; Harvey, P. D. New J. Chem.
1998, 247.
(8) Lloyd, B. R.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R. J.
Organometallics 1993, 12, 1231-1237.
(9) (a) Miedaner, A.; DuBois, D. L. Inorg. Chem. 1988, 27, 2479. (b)
Murahashi, T.; Nagai, T.; Okuno, T.; Matsutani, T.; Kurosawa, H.
Chem. Comm. 2000, 1689.
(10) Maekawa, M.; Munakata, M.; Kuroda-Sowa, T.; Suenaga, Y. Poly-
hedron 1998, 17, 3657.
(11) Hunt, C. T.; Balch, A. L. Inorg. Chem. 1982, 21, 1641.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3085

Cugnet et al.
Figure 4. (a) RDE voltammograms of [Pd3]2+ (scan rate ) 0.02 V s-1
)
in THF/Bu4NPF6 solution at room temperature and after (b) the addition of
1 equiv of Pd2Cl2, (c) 1-electron reduction at -0.8 V, (d) 2-electron
reduction at -0.8 V, and (e) 2.6-electron reduction at -0.8 V. Starting
potential: 0.2 V
can observe two signals at 982 (100%) and 1017 (6.5%)
+
associated to fragments Pd2(dppm)2 and Pd2(dppm)2Cl+.
Coulometric measurements at peak N (for the adduct [Pd3-
(Cl)]+) do not provide the expected transfer of 2 electrons
as suggested by eq 4, but rather, values varying between
2.6 and 3 were obtained. An example of evolution of RDE
voltammograms is shown in Figure 4.
After a one-electron reduction, no oxidation wave in the
range potential of wave N is observed. However after
consumption of two electrons, a well-defined oxidation wave
N′ appears. Its current intensity increases as the quantity of
electricity consumed increases, even for a quantity of
electricity exceeding two electrons (see entries for 2.6
electrons). Moreover, the reduction wave A is still present,
but its current intensity is smaller than the initial wave prior
to electrolysis. One question that comes to mind is why are
the peaks A′1 and A′2 obtained in the CV (Figure 2, trace c)
but do not appear after the electrolysis (Figure 3). To answer
this question, the reactivity of Pd2Cl2 toward the electro-
generated paramagnetic [Pd3]+ and zerovalent species [Pd3]0
was investigated because no real affinity exists between these
low-valent species and Cl- anions.
Reactivity of [Pd3]+ toward Pd2Cl2. Figure 5 shows the
RDE voltammograms of the electrogenerated [Pd3]+ which
exhibits an oxidation wave, A′1 (reverse reaction of eq 1)
and a reduction wave, A2 (Figure 5, trace b). When one
equivalent of Pd2Cl2 is added, drastic modifications are
readily observed. Waves A′1 and A2 disappear and reduction
waves N1 and A appear (Figure 5, trace c). After a two
electron reduction at -0.9 V (plateau of wave N1), the
oxidation wave N1′, which is well defined, and reduction
wave A are observed (Figure 5, trace d). The reaction
is complete in [Pd3]+, but dimer Pd2Cl2 is still present
(wave A).
Figure 3. Space filling models for 12+ and 2 showing the screening of
the Cl ligand on 2 preventing good interactions with the Pd3 center. This
conformation is the one that shows the largest possible cavity size. This
global conformation depends on the dppm conformation.
complexes.5,12 Subsequently, the free Cl- ion is scavenged
by [Pd3]2+
[Pd3]2+ + Cl- f [Pd3(Cl)]+
(7)
To check this hypothesis, the reactivity of Pd2Cl2 in the
presence of AgPF6 in THF was also investigated. The 31P
NMR spectrum exhibits the same pseudo-triplets described
above, in agreement with the formation of [Pd2Cl]+.13 When
the reaction occurs in MeCN, two different pseudo-triplets
are also observed at 1.6 and -5.5 ppm. These values differ
from that observed for eq 4 described above (J ) 42.5 Hz).
Moreover this cationic species, prepared in THF or MeCN,
was characterized by MALDI-TOF mass spectrometry. We
When 0.5 equiv of Pd2Cl2 is used (instead of 1 equiv),
the reaction also reaches completion (disappearance of
[Pd3]+), and wave A disappears as well. In both cases, no
EPR signal is detected, confirming not only that [Pd3]+, a
paramagnetic cluster, is totally consumed but also that no
other paramagnetic species is formed. In addition, both
(12) For example, see: Kullberg, M. L.; Lemke, F. R.; Powell, D. R.;
Kubiak, C. P. Inorg. Chem. 1985, 24, 3589.
(13) When Ag+ (AgPF6) is added to the solution containing Pd2Cl2, a
reduction wave at -0.55 V appears, which corresponds to the reduction
of [Pd2Cl]+.
3086 Inorganic Chemistry, Vol. 46, No. 8, 2007

Pd-Cl Bond CleaWage
Figure 6. (a) RDE voltammograms of [Pd3]2+ (scan rate ) 0.02 V s-1
)
Figure 5. (a) RDE voltammograms of [Pd3]2+ (scan rate ) 0.02 V s-1
)
in THF/Bu4NPF6 solution at room temperature, (b) after a 2-electron
reduction at -1 V and (c) that shown in trace b after the addition of 1
equiv of Pd2Cl2, (d) that shown in trace c after evolution for 30 min, and
(e) that shown in trace c after 2.5 h. Starting potential: 0.2 V.
in THF/Bu4NPF6 solution at room temperature, (b) after 1-electron reduction
at -1 V producing [Pd3]+, and of the (c) solution from trace b after the
addition of 1 equiv of Pd2Cl2, (d) solution from c after a 1-electron reduction
at -0.9 V, and (e) Solution from c after a 2-electron reduction at -0.9 V.
Starting potential: 0.2 V.
reactivity between two paramagnetic species (probably very
fast)
UV-vis (characteristic band at 460 nm) and 31P NMR (δ )
-9.8 ppm) spectroscopy confirm the presence of [Pd3(Cl)]+.
Suspecting the presence of the highly reactive zerovalent
species such as Pd2(dppm)2 or Pd(dppm) (these species may
be in equilibrium, as mentioned for the related ligands dcpe
and dppe),14 we evaporated the crude product under Ar and
recorded the 31P NMR spectrum using CDCl3 to induce an
oxidative addition that produces Pd(dppm)Cl2. The reaction
of dihalomethanes with a binuclear palladium(0) complex
was reported.15 The other signal, in addition ot the resonance
at -9.8 ppm discussed above, is indeed at -56.3 ppm
(singlet) as verified with an authentic sample.11 The zerova-
lent species Pd2(dppm)316 formed according to reaction 8 was
detected by 31P NMR (δ ) 12.4 ppm) and electrochemistry
(oxidation wave at -0.23 V) after the addition of an excess
of dppm in solution.7
Pd2(dppm)2Cl(THF)+ + 2[Pd3]+ + Cl- f
Pd2(dppm)2(Cl)THF + [Pd3Cl]+ (10)
Pd2(dppm)2Cl(THF) + 2[Pd3]+ f
Pd2(dppm)2 + [Pd3Cl]+ + THF (11)
Pd2(dppm)2Cl(THF)+ + 2[Pd3]+ + Cl- f
Pd2(dppm)2 + THF + 2 [Pd3Cl]+ (12)
The driving force of the reaction is the great stability of
adduct [Pd3(Cl)]+.2d Stabilization of paramagnetic M-M
bonded dimers, notably for d6-d7 Pt2 systems, was observed
before.17
Reactivity of [Pd3]0 toward Pd2Cl2. Figure 6 exhibits the
RDE voltammogram of [Pd3]0 (generated from the 2-electron
reduction of [Pd3]2+). Two oxidation waves A′1 and A′2 are
present (trace b). After the addition of 1 equiv of Pd2Cl2,
waves A′1 and A′2 disappear slowly over a period of 30 min
(trace c). At this point, the reduction waves N and A and
the oxidation wave N′ are observed (trace d). Again, the
UV-vis and 31P NMR spectra confirm the presence of [Pd3-
(Cl)]+, and NMR evidence confirms that of Pd(dppm)Cl2,
after treatment of the crude product with CDCl3 (peak at
-56.0 ppm). In the presence of dppm, the reaction yields
Pd2(dppm)3 (31P NMR singlet at 12.4 ppm in acetone-d6 and
oxidation wave -0.23 V vs SCE confirmed with an authentic
sample).7
2[Pd3]+ + Pd2(dppm)2Cl2 + dppm f
2[Pd3Cl]+ + Pd2(dppm)3 (8)
The overall result can be interpreted by electron-ligand
exchange reaction 9
2[Pd3]+ + Pd2(dppm)2Cl2 f 2[Pd3Cl]+ + Pd2(dppm)2 (9)
The outer-sphere electron transfer between Pd2Cl2 and
[Pd3]+ is impossible on the basis of the redox potentials (the
difference of 0.84 V is too large). However, the cationic
13
dimer [Pd2Cl]+ should be more easily reduced than Pd2Cl2
and can be reduced by [Pd3]+. A possible mechanism can
be formulated to reactions 10 and 11 or 12. Equation 10
corresponds to a redox reaction, and 11 corresponds to the
In the presence of 0.5 equiv of Pd2Cl2, the reaction is not
quantitative and waves A′1 and A′2 are still observed even
after several hours. This process is summarized by eq 13
2[Pd3]0 + Pd2(dppm)2Cl2 f
(14) (a) Reid, S. M.; Fink, M. J. Organometallics 2001, 20, 2959. (b)
Landtiser, R.; Pan, Y.; Fink, M. J. Phosphorus, Sulfur Silicon Relat.
Elem. 1994, 93-94, 393.
(15) For examples, see: (a) Tsubomura, T.; Itsuki, A.; Homma, M.; Sakai,
K. Chem. Lett. 1994, 661. (b) Balch, A. L.; Hunt, C. T.; Lee, C.-L.;
Olmstead, M. M.; Farr, J. P. J. Am. Chem. Soc. 1981, 103, 3764.
(16) Lindsey, C. H.; Benner, L. S.; Balch, A. L. Inorg. Chem. 1980, 19,
3503.
[Pd3Cl]+ + Pd2(dppm)2 + Cl- (13)
(17) Bennett, M. A.; Bhargava, S. K.; Boas, J. F.; Boere´, R. T.; Bond, A.
M.; Edwards, A. J.; Guo, S.-X.; Hammerl, A.; Pilbrow, J. R.; Priver,
S. H.; Schwerdtfeger, P. Inorg. Chem. 2005, 44, 2472.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3087

Cugnet et al.
Scheme 1
Scheme 3
Scheme 2
Scheme 4
We suggest that the key step is an electron-ligand exchange
reaction4 taking place between the intermediate compound
[Pd2Cl]+ and the zerovalent cluster [Pd3]0
redox-active Ag+ cation, only the Pd2Cl+ species is obtained.
In addition, this latter reaction is stoichiometric only. These
new results suggest that halide abstraction from Pd2Cl2 can
be mediated by [Pd3Cl]+ which is electrochemically reduced
to [Pd3]0 at a much lower potential than that of Pd2Cl2
(Scheme 3). Useful electrochemically assisted catalytic
processes are clearly in sight in this area using [Pd3]2+ and
its related derivatives. In this particular example, the highly
reactive zerovalent Pd2(dppm)2 can be generated in situ
without using chemical18 or photochemical processes, at
relatively low reductive potentials (without reducing the
organics). This process involves the 2-electron reduction of
[Pd3Cl]+ to form the active species [Pd3]0 which reduces Pd2-
Cl2 as shown below.
The catalytic formation of [Pd2Cl]+ can be also obtained
using a sacrificial copper anode, as already mentioned in
the acylchloride activation, which involved the electrochemi-
cal regeneration of the [Pd3]2+ dication from [Pd3Cl]+ and
the precipitation of CuCl.3b,3d
This methodology is currently being tested using other
organometallic precursors along with the testing of organic
reactions mediated by the electrochemically assisted genera-
tion of the active intermediates.
[Pd3]0 + [Pd2(dppm)2(THF)Cl]+ f
[Pd3Cl]+ + Pd2(dppm)2 + THF (14)
Again, the stability of [Pd3(Cl)]+ must drive (as the binding
constant is very large)2d the reaction, and in this case, [Pd3]2+
mediates the reduction of Pd2Cl2.
As mentioned, coulometry reproducibly indicates a transfer
of more than 2 electrons for the [Pd3]2+ and Pd2Cl2 (between
2.6 and 3.0) using the reduction potential at wave N. This
can now be explained by the subsequent reactivity of [Pd3]0
with Pd2Cl2.
Clearly, the stronger Lewis acidity of [Pd3]2+ toward Cl-
drives the reaction between [Pd3]2+ and Pd2Cl2, while the
reducing ability of the paramagnetic species [Pd3]+ and
zerovalent cluster [Pd3]0 toward [Pd2Cl]+ is involved when
these reduced species are electrogenerated in situ. Scheme
1 summarizes the results.
The reactions describing the overall reactivities of [Pd3]2+,
[Pd3]+, and [Pd3]0 toward Pd2Cl2 are summarized in Scheme
2.
Conclusion
Acknowledgment. P.D.H. thanks NSERC (Natural Sci-
ences and Engineering Research Council of Canada) for
funding. Y.M. is grateful to CNRS (Centre National de la
Recherche Scientifique) for funding. We are grateful to Dr
Jean-Michel Barbe (LIMBRES, Dijon) for MALDI-TOF
experiments.
This halide-abstraction reactivity from an inorganic mol-
ecule by the strong Lewis acid [Pd3]2+ (which is known for
C-X bond cleavage) is new. Indeed, in the absence of direct
2+
interactions of the Pd-Cl fragment of Pd2Cl2 with the Pd3
center inside the cavity because of steric interactions indicates
that the mechanism proceeds via a dissociative mechanism
where the uncoordinated Cl- ion is competitively abstracted
Supporting Information Available: MALDI-TOF spectrum of
[Pd2(dppm)2(THF)(Cl)]+. This material is available free of charge
via the Internet at http://pubs.acs.org.
2+
by Pd3 rather than Pd2(dppm)2Cl+ (back reaction). This
reactivity also operates for its corresponding reduced forms
([Pd3]+ and [Pd3]0) for which the strong stability of [Pd3-
Cl]+ drives the reactions. Notably, in the case of the non-
IC061777H
(18) Pan, Y.; Mague, J. T.; Fink, M. J. J. Am. Chem. Soc. 1993, 115, 3842.
3088 Inorganic Chemistry, Vol. 46, No. 8, 2007