Reactivity and electrochemical properties of the d8-d 8 Pd2(dppm)2(Cl)2(Me)2 towards MeI

Article abstract of DOI:10.1016/S0020-1693(02)01554-2

The title dimer reacts with MeI or KI to form [Pd₂(dppm) ₂(μ-I)(Me)₂]I. The latter has been characterized from X-ray crystallography, and exhibits a A-frame structure with a Pd?Pd separation of 3.061 A?. The 4-electron red

Full text of DOI:10.1016/S0020-1693(02)01554-2

Inorganica Chimica Acta 350 (2003) 442ꢀ448
/
www.elsevier.com/locate/ica
Reactivity and electrochemical properties of the d8ꢀ
Pd2(dppm)2(Cl)2(Me)2 towards MeI
/
d8
David Evrard a,b, Marc Drouin a, Yves Mugnier b,*, Pierre D. Harvey a,*
a De´partement de Chimie, Universite´ de Sherbrooke, Sherbrooke, PQ, Canada J1K 2R1
´
Laboratoire de Synthe`se et d’Electrosynthe`se Organome´talliques (LSEO-UMR 5632), Faculte´ des Sciences Gabriel, 6 boulevard Gabriel, 21000 Dijon,
France
b
Received 12 July 2002; accepted 7 October 2002
This paper is dedicated to Professor Pierre Braunstein
Abstract
The title dimer reacts with MeI or KI to form [Pd2(dppm)2(m-I)(Me)2]I. The latter has been characterized from X-ray
8
Pd separation of 3.061 A. The 4-electron reduction of the d ꢀ
d8 title
/
˚
crystallography, and exhibits a A-frame structure with a Pd
starting material Pd2(dppm)2(Cl)2(Me)2 and resulting [Pd2(dppm)2(m-I)(Me)2]I product in the presence of dppm, produces the
known Pd2(dppm)3 dimer. The presence of intermediates such as the d9ꢀd9 ‘Pd2(dppm)2(Me)2’ and d10 ꢀd10 ‘Pd2(dppm)2(I)nnꢁ
species are suspected, and 31P NMR indicates evidence for the relatively unstable mononuclear Pd(dppm)(Me)2 complex. The
reductive electrolysis of both d8ꢀd8 dimers in the presence of an excess of MeI leads to an increase in Pd(dppm)(Me)2 concentration.
/
Á Á Á
/
/
/
’
/
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Electrolyses; Dinuclear; Electrochemical; X-ray crystallography
1. Introduction
In that context, the common approach is the
Grignard-type electrochemical reaction (RMgXꢃ
CO2ꢃ
MgXꢁꢃRCO2ꢁ) [10]. Such an approach
/
/
2eꢁ 0
/
/
Palladium and its coordination complexes and clus-
ters are well known for their importance in hetero- and
involves the anchoring of organic fragments and CO2
simultaneously onto the Pd centre (or centres). Appro-
homogeneous catalysis [1ꢀ
investigated a number of stoichiometric and catalytic
electrochemical reactions such as CO2 activation [5,6],
/
4]. Recently our groups have
priate candidate inorganic species are the d8ꢀ
/
d8 either as
25] structures.
face-to-face [11ꢀ
/
15] or A-frame [16ꢀ
/
Previous works have shown that electrochemical reduc-
tion of Pd(dppm)Cl2 and Pd2(dppm)2Cl2 leads to the
Cꢀ
formate decomposition [8], using various species of the
type ‘Pdx(dppm)x’ such as Pd(dppm)Cl2,
Pd2(dppm)2Cl2, and
Pd3(dppm)3(CO)2ꢃ
Pd4(dppm)4(H)22ꢃ (Scheme 1). With respect to CO2
activation, while the electrochemical reductive coupling
of Hꢃ and CO2 to generate formate ion is well known,
/
X bond cleavage (Xꢂ
/
Br, I) [7], H2-evolution and
reactive zerovalent species Pd(dppm)l/2
/
1Pd2(dppm)2 [6],
,
which are responsible for CO2 activation. The latter
species has been observed earlier at low temperature by
Stille et al. [26] by NMR spectroscopy, but no X-ray
data have been available in this case. On the other hand,
at least two other examples of zerovalent Pd2(diphos)2
species have recently been characterized from X-ray
the Cꢀ
/
C bond formation between CO2 and an organic
fragment is more rare [9].
crystallography
(diphosꢂ/Cy2PCH2PCy2
[15],
iPr2PCH2PiPr2 [27]). In addition, unsaturated zerova-
lent Pd species are readily anticipated to undergo an
oxidative addition towards halocarbons. In that respect,
the Pd2(dppm)2 species should react with MeI prior to
the electrochemical event. Years ago, Balch et al. [28]
* Corresponding authors. Tel./fax: ꢃ
1-819-821 7092; fax: ꢃ1-819-821 8017 (P.D.H.).
E-mail addresses: yves.mugnier@u-bourgogne.fr (Y. Mugnier),
pharvey@courrier.usherb.ca (P.D. Harvey).
/
33-3-80-39 6091 (Y.M.); Tel.:
ꢃ/
/
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01554-2

D. Evrard et al. / Inorganica Chimica Acta 350 (2003) 442ꢀ
/448
443
Scheme 1.
reported the reactivity of the related Pd2(dppm)3 dimer
2. Chemical reactivity
(potential precursor of Pd2(dppm)2) with MeI. A species
correctly formulated as Pd2(dppm)2(I)2(Me)2 was re-
ported, and used to prepare the salts [Pd2(dppm)2(m-
Pd2(dppm)2(Cl)2(Me)2 reacts with MeI to form
[Pd2(dppm)2(m-I)(Me)2]I (Eq. (1)) as unambiguously
identified from X-ray crystallography (Fig. 1):
I)(Me)2]X with Xꢁ ꢂPF6ꢁ and BF4ꢁ, assigned on the
/
basis of NMR findings. Based on the Cambridge Data
Bank none of theses species have been characterized
crystallographically. In the light of the literature data
and the electrochemical experiments described above,
the reactivity of Pd2(dppm)2(Cl)2(Me)2 complex with
MeI and KI has been investigated. We now wish to
report some preliminary findings on the electrochemical
behaviour of Pd2(dppm)2(Me)2Cl2 in the presence of
MeI and CO2. During the course of this study, the
crystal structure of the A-frame dimer [Pd2(dppm)2-
(Me)2(m-I)]ꢃ has been obtained (Table 1).
Pd2(dppm)2(Cl)2(Me)2 ꢃ2MeI
0 [Pd2(dppm)2(m-I)(Me)2]Iꢃ2MeCl(g)
(1)
The reaction of Pd2(dppm)2(Me)2Cl2 with MeI is
intriguing since it highlights an unsuspected reactivity;
Table 1
Crystallographic data for [Pd2(dppm)2(Me)2(m-I)]I×2CHCl3
/
Crystal system
Space group
monoclinic
P21/n
˚
a, b, c (A)
10.113(2), 22.430(5), 27.001(5)
90.00, 92.34(3), 90.00
a, b, g (8)
Cell volume (A )
3
˚
6120(2)
4
Z
D (mg mꢁ3
Temperature
)
1.633
293(2)
16.318
Absorption coefficient (mmꢁ1
)
u range (8)
2.56ꢀ69.88
/
Reflections collected
Independent reflections
Observed reflections
Absorption correction
No. parameters refined
11 020
11 020
3500 (Fꢀ
none
614
/
2.0s(F))
R(F), Rw(F2), (F2ꢀ
/
2.0s(F2))
0.0773, 0.1872
0.1744, 0.2230
0.724
R(F), Rw(F2) (all data)
Goodness-of-fit
Fig. 1. ORTEP drawing for [Pd2(dppm)2(m-I)(Me)2]I. The ellipsoids
are shown at 30% probability. The H-atoms are not shown for clarity.

444
D. Evrard et al. / Inorganica Chimica Acta 350 (2003) 442ꢀ
/448
Table 2
Comparison of Pd
/
Á Á Á
/
Pd distances in X-bridged A-frame species
ꢁ1
˚
Pd) A
Compounds
d(Pd
/
Á Á Á
/
References
[Pd2(dppm)2(Me)2(m-I)]I
[Pd2(dppm)2(Me)(I)(m-I)]BF4
3.061(2)
2.974(6)
3.015(10)
3.356(1)
3.378
this work
[28,29]
Scheme 2.
[Pd2(dppm)2(Bz)2(m-Br)]PF6
[Pd2(dppm)2(Mes)2(m-Br)]PF6
[Pd2(dppm)2(Cl)2(m-Cl)]OH
[Pd2(dppm)2(H)(Me)(m-Cl)]BPh4
[30]
[31]
[30]
[26]
oxidative addition of halocarbons on Pd(II) centres is
uncommon. However, Balch et al. [28] have long
demonstrated the equilibria in solution between halide-
bridged and unbridged Pd2 dimers as shown in Scheme
2.
The Clꢁ ion is not reknown for its nucleophility, but
the Iꢁ is a good leaving group in MeI, and can be
captured by the intermediate [Pd2(dppm)2(m-Cl)(Me)2]ꢃ
to form a second intermediate ‘[Pd2(dppm)2(Cl)(I)-
(Me)2]’. The latter species leads to the observed product
upon a loss of the labile Clꢁ ion. No trace of Clꢁ ion
was seen in these specific experiments, and the unob-
served but volatile organic CH3Cl side product is
suspected in this case. In order to support this proposal,
Pd2(dppm)2(Me)2Cl2 was reacted with KI, and the m-I
A-frame product was obtained quantitatively (Eq. (2)):
3.170
3.013(1)
X
ꢁ ꢂClꢁ, Brꢁ, and Iꢁ, respectively, at room tem-
perature.
/
3. X-ray structure
This [Pd2(dppm)2(m-I)(Me)2]ꢃ complex exhibits the
well-known A-frame structure where the Me groups
occupy the terminal positions, while the iodide ion
bridges the two metal atoms. The long Pd
/
Á Á Á
tion (3.061(2) A) indicates the absence of Pdꢀ
bonding, expected for the d8ꢀd8 electronic configura-
tion. The comparison of the Pd Pd distances for
Á Á Á
various halogen-bridged A-frame species [26,28ꢀ31] is
interesting (Table 2). First the dissymmetric species
exhibits approximately similar Pd Pd separations to
Á Á Á
the symmetric ones for a same X-bridge. Second the Pd
Á Á ÁPd distances do not follow an expected trend, but
vary as BrꢀCl$I, based on the only data available.
/Pd separa-
˚
/Pd
/
/
/
Pd2(dppm)2(Cl)2(Me)2 ꢃ2KI
/
0 [Pd2(dppm)2(m-I)(Me)2]Iꢃ2KCl(s)
(2)
/
/
Scheme 3 summarizes the mechanism (Scheme 3).
/
Parallel studies show that addition of KF does not
change the 31P NMR spectra of Pd2(dppm)2(Cl)2(Me)2,
indicating the lack of reactivity. Addition of KCl
induces only a very small shift of the 31P NMR signal,
consistent with the undissociated nature of the starting
material. Addition of KBr induces changes in the 31P
and 1H NMR spectra that are consistent with the
presence of both the dissociated ([Pd2(dppm)2(m-
Br)(Me)2]ꢃ) and undissociated species (Pd2(dppm)2-
(Br)2(Me)2) [28]. These experiments indicate that the
Pd2(dppm)2(X)2(Me)2 species exist primarily as undis-
sociated, undissociated/dissociated, and dissociated for
/
/
/
Difference in inductive effect cannot be invoked in this
case since the Bz and Me groups behave similarly in that
respect. In addition, steric hindrance does not seem to
play an important role since the d(Pd
/
Á Á Á
/
Pd) data for
[Pd2(dppm)2(Bz)2(m-Br)]ꢃ and [Pd2(dppm)2(Mes)2(m-
Br)]ꢃ are practically identical.
Based upon the sum of the covalent radii for Pd
(rcovÂ/12 of a typical Pdꢀ
(rion
exhibit a significant covalent character, but are longer
/
Pd bond Â
/
2.6 A [32]) and I
˚
˚
˚
ꢂ
/
1.34 A [33]) the bridging Pdꢀ
/
I bonds (Â2.74 A)
/
than that found for the terminal PdꢀI bonds (2.55 and
/
ꢃ
˚
2.58 A) in [Pd2(dppm)2(Me)(I)(m-I)] [29]. In the latter
case the bridging PdꢀI distances are also long (2.648ꢀ
2.740 A). This trend (d(PdꢀX) bridgingꢀd(PdꢀX)
terminal) is seen for all complexes. The PdꢀC and
P bond lengths, and angles, are found normal
/
/
˚
/
/
/
/
Pdꢀ
/
(Table 3).
4. Electrochemical properties
The rotating disk electrode (RDE) voltammogram of
[Pd2(dppm)2(m-I)(Me)2]I exhibits a reduction wave (A1)
at ꢁ
/
1.1 V, and two oxidation waves (I1? and I2?) located
Scheme 3.
at 0.45 and 0.8 V vs saturated calomel electrode (SCE),

D. Evrard et al. / Inorganica Chimica Acta 350 (2003) 442ꢀ
/448
445
Table 3
Selected bond distances (A) and angles (8)
best formulated as ‘Pd2(dppm)2(I)nnꢁ’. The oxidation
wave associated to the presence of Iꢁ (I1?) no longer
˚
appears. Presumably nꢂ2 here. Such species have
/
Pd1ꢀ
Pd1ꢀ
Pd2ꢀ
Pd1ꢀ
Pd2ꢀ
/
Pd2
3.0607(16)
2.723(2)
2.755(2)
Pd1ꢀ
Pd1ꢀ
Pd2ꢀ
Pd2ꢀ
/
P1
P3
P4
P2
2.387(4)
2.397(4)
2.393(4)
2.405(4)
already been postulated in the case of reduction of the
related Pd(dppm)Cl2 and Pd2(dppm)2Cl2 complexes [6].
In the presence of added dppm (1.1 equiv.), the RDE
and CV data are identical to that above, but one
important difference is noticed after a 2-electron reduc-
/I
/
/I
/
/C4
2.154(13)
2.222(14)
/
/C3
Pd1ꢀ
IꢀPd1ꢀ
IꢀPd2ꢀ
IꢀPd1ꢀ
IꢀPd2ꢀ
P1ꢀ
P4ꢀ
/
Iꢀ
/Pd2
67.94(5)
173.4(4)
175.6(4)
56.53(4)
55.53(4)
176.79(14)
178.66(14)
C4ꢀ
C4ꢀ
C3ꢀ
C3ꢀ
P3ꢀPd1ꢀ
P1ꢀPd1ꢀ
P4ꢀPd2ꢀ
P2ꢀPd2ꢀ
/
Pd1ꢀ
Pd1ꢀ
Pd2ꢀ
Pd2ꢀ
/
P3
P1
P2
P4
89.5(4)
87.8(4)
86.9(4)
/
/C4
/
/
tion (nexp
ꢂ
/
1.9). Indeed the E? wave is replaced by the
/
/C3
/
/
wave Eƒ at ꢁ
/
0.16 V (Fig. 3, trace b). This signal is
/
/Pd2
/
/
92.1(4)
/
/Pd1
/
/
Pd2
Pd2
Pd1
Pd1
99.90(10)
83.19(10)
82.86(10)
98.47(10)
characteristic of the oxidation of Pd2(dppm)3 as verified
with an authentic sample [6]. The 31P NMR spectra of
/
Pd1ꢀ
/
P3
/
/
/
Pd2ꢀ
/
P2
/
/
the bulk electrolysis also exhibit a peak at ꢁ26.5 ppm
/
/
/
which can be associated to a mononuclear complex
Pd(dppm)(Me)2, and the characteristic signal of
Pd2(dppm)3 at ꢃ
based upon the related mononuclear Pd(dppm)R2 com-
plexes for which ꢁ32B 26 ppm. This
31P NMRB
compound is also known to be too unstable to be
isolated [31,35]. The subsequent reduction at wave B2 of
the resulting solution in the presence of dppm leads to
Pd2(dppm)3, after the consumption of 2 F molꢁ1
/
14.6 ppm. This former assignment is
respectively, (Fig. 2, trace a). The latter signals are the
bielectronic oxidations of Iꢁ as verified with Bu4NI [34]
(i.e., the presence of Iꢀ (I1?) and the oxidation product
I3ꢀ (I2?)), and are consistent with the ionic nature of the
reactant. The CV exhibits the same processes, but
additionally, a second reduction wave B is observed at
/
/d
/
ꢁ
/
ꢁ2.5 V vs SCE. This species responsible for this
/
(nexp
complex is interpreted according to a reaction scheme
ꢂ1.8) (Fig. 3, trace c). The overall results for this
/
reduction peak is assigned to a non-containing halide
complex (see below) but no secure assignment can be
provided at the moment.
presented in Scheme 4.
The postulated electrogenerated ‘Pd2(dppm)2(Me)2’
intermediate dismutes to form slowly two other inter-
Bulk electrolysis at ꢁ
/
1.3 V vs SCE is completed after
a consumption of 2 F molꢁ1 (nexp
product exhibits a RDE voltammogram with an oxida-
ꢂ/2.08). The resulting
mediates formulated as ‘Pd2(dppm)2(I)nnꢁ
’
and
Pd(dppm)(Me)2. The first dismutation product
‘Pd2(dppm)2(I)nnꢁ’ is issued from an interaction between
1
tion wave at E/ ꢂ
/
ꢁ
/
0.05 V (signal E?), and two reduction
/2
1
waves at E/ ꢂ
/
ꢁ
/
1.4 and ꢁ1.5 V (B1 and B2, respec-
/
/2
‘Pd2(dppm)2’ and free Iꢁ ions. The proposed d9ꢀd9
/
tively), as shown in Fig. 2 (trace b). In the initial CV,
these three signals are not present, which indicates that
the species responsible for these new peaks appears only
slowly at the electrolysis time scale. One of the species is
oxidized at peak E?, which is characteristic of a species
intermediate ‘Pd2(dppm)2(Me)2’ was previously investi-
gated by Stille et al. [26], and was found too unstable to
be isolated. Instead this compound dismutes to for a d8
and d10 species, one of which was identified as
Fig. 3. RDE voltammogram of [Pd2(dppm)2(m-I)(Me)2]I on a carbon
electrode in a THF solution containing 0.2 mol lꢁ1 of Bu4NPF6 as
supporting electrolyte. Scan rate: 20 mV sꢁ1. (a) Before electrolysis;
Fig. 2. RDE voltammogram of [Pd2(dppm)2(m-I)(Me)2]I on a carbon
electrode in a THF solution containing 0.2 mol lꢁ1 of Bu4NPF6 as
supporting electrolyte. Scan rate: 20 mV sꢁ1. (a) Before electrolysis;
(b) after electrolysis at ꢁ
/
1.3 V and nꢂ
/
1.9eꢁ; (c) after second
(b) after electrolysis at ꢁ
/
1.3 V and nꢂ
/
2eꢁ
.
electrolysis at ꢁ1.6 V and nꢂ
/
/
1.8eꢁ
.

446
D. Evrard et al. / Inorganica Chimica Acta 350 (2003) 442ꢀ448
/
Scheme 4.
‘Pd2(dppm)2’, consistent with this work’s findings. Then,
the trapping of the d10ꢀd10 species ‘Pd2(dppm)2(I)nnꢁ
/
’
proceeds according to a presumable substitution reac-
tion, where the weakly bound Iꢁ ions are displaced by
the stronger dppm ligand.
Fig. 4. Cyclic voltammogram of [Pd2(dppm)2(m-I)(Me)2]I on a carbon
electrode in a THF solution containing 0.2 mol lꢁ1 of Bu4NPF6 as
supporting electrolyte. Scan rate: 100 mV sꢁ1. (a) ꢃ
/
0.5 equiv. MeI; (b)
The second dismutation product Pd(dppm)(Me)2,
which exhibits reduction peaks at B1 and B2, is too
unstable to be isolated as stated, but could easily be seen
in the 31P NMR spectra. Its subsequent 2-electron
reduction in the presence of dppm leads to the forma-
tion of Pd2(dppm)3, hence releasing 2 Meꢁ carbanions.
The overall reaction in the case where free dppm is
added to the solutions, is:
ꢃ
/
1 equiv. MeI; (c) ꢃ
/
1.5 equiv. MeI; (d) ꢃ
/
1.5 equiv. MeI and CO2
bubbling.
[Pd2(dppm)2(m-I)(Me)2]Iꢃ2eꢁ ꢃ2MeI
0 2‘Pd(dppm)(Me)2’ꢃIꢁ3 ꢃIꢁ
(5)
The increase in intensity of waves I1? and I2? with
[MeI] clearly demonstrates that the formation of Iꢁ and
I3ꢁ (respectively) occurs during the reduction process.
Moreover, the increase in current intensity of B1 and
B2 when adding MeI suggests the possibility of a
catalytic process. Contrary to the electrolysis at the
potential of peak B2 without MeI that consume a
quantity of electricity of approximately 2 F molꢁ1, the
consumption of electricity becomes more important
(more than one electron per mole of MeI) for the
electrolysis performed in the presence of an excess of
MeI.
[Pd2(dppm)2(m-I)(Me)2]Iꢃdppmꢃ4eꢁ
0 Pd2(dppm)3 ꢃ2Iꢁ ꢃ2Meꢁ
(3)
The CV of Pd2(dppm)2(Cl)2(Me)2 exhibits two reduc-
tion peaks A2 and B which are located at ꢁ1.3 and ꢁ2.5
/
/
V vs SCE, respectively. The fact that peak B is located at
the same potential as for [Pd2(dppm)2(m-I)(Me)2]I in-
dicates that the electroreduction products in both cases
do not contain halide. The bulk electrolysis of
Eqs. (3) and (4) indicate the formation of the highly
reactive Meꢁ carbanion. This species is anticipated to
be a nucleophile towards CO2. When CO2 is bubbled
through a solution of [Pd2(dppm)2(m-I)(Me)2]I or
Pd2(dppm)2(Cl)2(Me)2 containing MeI, the CV exhibits
an unperturbed A1 peak, but shows an important
increase of the peaks B1 and B2, according to a probable
catalytic activation of CO2 (Fig. 4, trace d). Free CO2 in
Pd2(dppm)2(Cl)2(Me)2 at ꢁ
/
1.4 V vs SCE in the presence
of dppm consumes 4 F molꢁ1 and provides again, the
d10ꢀd10 dimer species Pd2(dppm)3 (based upon the CV
/
and 31P NMR data). The reduction proceeds according
to:
Pd2(dppm)2(Cl)2(Me)2 ꢃdppmꢃ4eꢁ
0 Pd2(dppm)3 ꢃ2Clꢁ ꢃ2Meꢁ
(4)
DMF is reduced at ꢁ2.1 V vs SCE [36].
/
The addition of MeI in a solution containing
Pd2(dppm)2(Cl)2(Me)2, leads to the formation of
[Pd2(dppm)2(m-I)(Me)2]I (as indicated in Eq. (1)). This
reaction is well seen in the RDE, where the characteristic
A1, I1? and I2? peaks are readily noted. Upon further
addition of MeI, peaks B1 and B2 appear, and increases
proportionally with the amount of MeI added (Fig. 4).
The 2-electron reduction of [Pd2(dppm)2(m-I)(Me)2]I in
the presence of MeI leads to a single product, which
exhibits only peaks B1 and B2. This CV signature is
indicative of the formation of Pd(dppm)(Me)2, and
allows to write the following equation:
5. Concluding remarks
The preliminary results indicate that the d8ꢀ
compound exhibits a reactivity towards MeI to produce
/
d8 title
the A-frame product [Pd2(dppm)2(m-I)(Me)2]I. Both
products can be electrochemically reduced to the d10ꢀ
/
d10 Pd2(dppm)3 dimer, when in the presence of free
dppm. This reduction reaction is suggestive of the
production of the Meꢁ carbanion, potential reactant
for CO2, en route towards acetate. Furthermore, the

D. Evrard et al. / Inorganica Chimica Acta 350 (2003) 442ꢀ
/448
447
suggestion of the presence of a catalytic process in Eq.
(5) leaves some hope that carboxylates can be synthe-
sized catalytically from organic halides and CO2. The
investigations of Pd2(dppm)2(X)2(Me)2 complexes
considerations, a statistical analysis of intensity distri-
bution, and the successful solution and refinement of the
structure. The CAD4-PC program [38] was used for
centering, indexing, and data collection. Two standard
reflections were measured every 60 min.
(Xꢁ ꢂ
/
Clꢁ, Brꢁ, Iꢁ; Rꢂ
various alkyls and aryls) are
/
in progress.
The NRCVAX programs [39] were used for crystal
structure solution by application of direct methods. The
SHELXL-97 program [40] was used for refinement by full-
matrix least-squares on F2. The absorption coefficients
(r) are listed in Table 1. Psi scan based empirical
absorption corrections were made. No significant decay
was observed during data collection. Isotropic extinc-
tion coefficients were included in the refinement to
account for secondary extinction effects [41]; their values
are given in Table 1.
6. Experimental
All reactions were carried out under an argon atmo-
sphere. Pd2(dppm)2(Cl2)(Me)2 has been prepared ac-
cording to literature procedure [26,28]. The solvents
CH2Cl2, tetrahydrofuran, acetone and pentane were
purchased from Fischer and were purified according to
Ref. [37]. 1H and 31P{1H} NMR spectra were measured
on a Bruker WM 300 spectrometer (1H NMR: 300.15
MHz, 31P NMR: 121.497 MHz). The reference was the
residual non-deuterated solvent. The chemical shifts are
reported with respect to TMS (1H NMR) and H3PO4
(31P NMR).
Hydrogen atoms were all geometrically placed and the
respective final refinements included anisotropic thermal
parameters for the non-hydrogen atoms, and isotropic
thermal parameters for the hydrogen atoms. Individual
displacement parameters were fixed at U(H)ꢂ
/
1.5ꢄ
/
Ueq
(C-methyl, OH) or U(H)ꢂ1.2ꢄUeq (C and N) and
/
/
were treated as rigid for refinement. The R(F) and
Rw(F2) final discrepancy indices at convergence for the
6.1. Synthesis of [Pd2(dppm)2(Me)2(m-I)]I
Inet
restraints and variables, and the GoF are listed in Table
1.
ꢂ2.0s(Inet) significant reflections, the number of
/
Method 1: to Pd2(dppm)2(Me)2Cl2 (100 mg, 0.092
mmol) dissolved in 50 ml of THF were added 50 ml
(0.803 mmol) of MeI. After 12 h, the stirring is stopped
in order to allow precipitation of [Pd2(dppm)2(Me)2I]I.
The beige precipitate was then filtered and washed twice
with 20 ml of ether. Yield: 96.4%.
[Pd2(dppm)2(m-I)(Me)2]I×
/
2CHCl3 gives crystals be-
longing to the monoclinic space group P21/n and at
˚
˚
293(2) K, aꢂ
/
10.113(2) A, bꢂ
27.001(5) A and bꢂ92.34(3), Vꢂ
R(F)ꢂ
0.0773, Rw(F2)ꢂ
/
22.430(5) A, cꢂ
/
3
˚
˚
/
/
6120(2) A , Zꢂ
/8,
Method 2: to Pd2(dppm)2(Me)2Cl2 (100 mg, 0.092
mmol) dissolved in 50 ml of acetone were added 150 mg
(0.903 mmol) of KI. After 12 h stirring, the solvent was
removed under vacuum and the beige solid was washed
twice with 20 ml of ether. Yield: 90%. Chem. Anal.;
Calc. for C52H50P4I2Pd2: C 49.35, H 3.98%; Found: C
/
/
0.1872. The disordered atoms
were considered as isotropic for refinement. Several
SAME restraints available in SHELXL were used during
the refinement to avoid unrealistic geometries.
6.1.2. Electrochemical experiments
1
48.70; H 3.99%. H NMR (CDCl3): 0.81 (m, 6H, Me),
4.51 (m, 4H, PCH2P), 7.40ꢀ
7.53 (m, 40H, Ph). 31P{1H}
All manipulations were performed 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 concentra-
tion of 0.2 M. For cyclic voltammetry experiments, the
concentration of the analyte was nearly 10ꢁ3 M.
Voltammetric analyses were carried out in a standard
three-electrode cell with a Tacussel UAP4 unit cell. The
reference electrode was a SCE separated from the
solution by a sintered glass disk. The auxiliary electrode
was a platinum wire. For all voltammetric measure-
ments, the working electrode was a vitreous carbon
/
NMR (CDCl3): 17.85 (s). FAB (NBA): 1139 (100%)
Pd2(dppm)2(Me)2I, 1251 (2%) Pd2(dppm)2(Me)I2, 1265
(1%) Pd2(dppm)2(Me)2I2. FT-Raman (solid) 500.7
(n(PdC)), 161.3 (n(PdP)), 118.8 cmꢁ1 (n(PdI)).
6.1.1. Crystallography
The A-frame compound [Pd2(dppm)2(m-I)(Me)2]I was
crystallized by slow vapor diffusion of hexane into a
CHCl3 solution, from which long brown crystals were
obtained. Intensity data from the long crystal were
collected at 293(1) K on an Enraf-Nonius CAD-4
automatic diffractometer. Table 1 provides crystallo-
graphic and data collection details.
electrode (fꢂ
ing in THF, the formal potential for the ferroceneꢃ/ꢁ
couple is found to be ꢃ0.56 V vs SCE. The controlled
/3 mm). In these conditions, when operat-
/
Cell constants and an orientation matrix for data
collection, were obtained from a least-squares refine-
ment using the setting angles of 24 centered reflections
potential electrolysis was performed with an Amel 552
potentiostat coupled with a Tacussel IG5-N integrator.
High scale electrolyses were performed in a cell with
three compartments separated with fritted glasses of
medium porosity. A carbon gauze was used as the
in the range 408B
/
2u B508. The space group determina-
/
tion was based upon systematic absences, packing

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displacement parameters, (an)isotropic displacement
parameters, bond distances, bond angles, torsion angles,
contact distances, hydrogen bonds (36 pages) are avail-
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Acknowledgements
P.D.H. thanks the NSERC (Natural Sciences and
Engineering Research Council) and FCAR (Fonds
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