Synthesis and characterization of binuclear palladium(I) compounds and the influence of competing arenes

Article abstract of DOI:10.1016/j.jorganchem.2010.03.011

The binuclear palladium(I) compounds, [Pd₂(Ga₂Cl₇)₂(C₇H₈)₂] (1), [Pd₂(GaCl₄)₂(C₉H₁₂)₂]·C₉H₁₂

Full text of DOI:10.1016/j.jorganchem.2010.03.011

Journal of Organometallic Chemistry 695 (2010) 1513–1517
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of binuclear palladium(I) compounds
and the influence of competing arenes
*
Josefin Åkerstedt, Mikhail Gorlov, Andreas Fischer, Lars Kloo
Division of Inorganic Chemistry, Royal Institute of Technology, Stockholm S-100 44, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
The binuclear palladium(I) compounds, [Pd2(Ga2Cl7)2(C7H8)2] (1), [Pd2(GaCl4)2(C9H12)2]ꢀC9H12 (2) and
[Pd2(Ga2Cl7)2(C6H5Cl)2] (3), have been prepared from palladium(II) chloride in gallium(III) chloride-arene
reaction media. All isolated crystalline solids (1, 2 and 3) have been structurally characterized by single
crystal X-ray diffraction and Raman spectroscopy. The results form quantum chemical calculations on the
interaction energies of the arenes and the dipalladium unit of these compounds is also presented.
Ó 2010 Elsevier B.V. All rights reserved.
Received 27 January 2010
Received in revised form 26 February 2010
Accepted 4 March 2010
Available online 10 March 2010
Keywords:
Palladium
Gallium chloride
Arenes
Binuclear compounds
Crystal structure
1. Introduction
Furthermore, in recent years several one- and two-dimensional
chains and networks of polynuclear palladium compounds, so
Gallium halide-arene mixtures have been shown to be powerful
tools, capable of both reducing metal centers and acting as halide
scavengers [1]. It is the special properties of gallium(III) halides
to be extremely soluble in arenes (up to 50 mol% in benzene), the
ability to accept halide ions from metal halides and the reducing
ability of GaX3-arene mixtures that make them suitable for the
preparation of low-valent clusters and compounds of the transition
metals [2]. In the formation of binuclear transition metal com-
pounds with coordinating arenes these properties are central,
along with the nature of the arene-ring substituents and transition
metal centers, affecting the structure of the metal-arene com-
pounds formed.
The first binuclear palladium compounds having arenes as li-
gands were prepared in an AlCl3/Al mixture by Allegra et al. [3].
In these reactions, elemental aluminum was proposed as the
reducing agent, reducing palladium(II) to palladium(I). It was later
shown in our group that a GaCl3/arene mixture not including
metallic gallium enable the formation of binuclear palladium(I)
compounds under mild conditions. It is believed that the reduction
of the palladium(II) center occurs via oxidation (i.e. halogenation)
of the arenes [4a]. A number of binuclear arene-supported palla-
dium(I) compounds have been prepared in this way in our lab [4].
called ‘sandwich compounds’, coordinating different polycyclic
aromatic hydrocarbons (PAHs) have been reported by Kurosawa
and co-workers [5]. Most of these compounds have been prepared
through the use of the substitutionally labile homoleptic acetoni-
trile dipalladium compound [Pd2(CH3CN)6][BF4]2.
Palladium, along with its neighboring Group 10 metals, plays a
central role in the field of catalysis [6]. The Group 10 metals are
indispensible for organic synthesis, playing a part as catalysts in
reactions such as carbonylation [7], isomerization [8] and dimeri-
sation [9]. Palladium catalysed carbon–carbon bond forming reac-
tions have been especially significant to the development of
organic chemistry and are valuable in industry. The catalytic prop-
erties of palladium explain the general interest in palladium clus-
ters and compounds, exhibiting changes in coordination and
oxidation state.
The aim of this study has been to use the exceptional reductive
ability of the gallium (III) halide-arene mixtures in quest for new
polynuclear palladium compounds supported by simple arenes or
polyaromatic hydrocarbons, such as naphthalene, anthracene or
biphenyl.
In this work we report the syntheses and crystal structures
of three new binuclear palladium(I) compounds; [Pd2(Ga2Cl7)2-
(C7H8)2] (1), [Pd2(GaCl4)2(C9H12)2]ꢀC9H12 (2), [Pd2(Ga2Cl7)2-
(C6H5Cl)2] (3), one of which was solely recovered from a mixed
arene reaction medium, highlighting the effects of arene competi-
tion with respect to coordination to the Pd22þ unit.
* Corresponding author. Tel.: +46 8 7909343; fax: +46 8 7909349.
E-mail address: larsa@kth.se (L. Kloo).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.03.011

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J. Åkerstedt et al. / Journal of Organometallic Chemistry 695 (2010) 1513–1517
2. Results and discussion
The binuclear palladium–toluene compound was obtained from
the straightforward room-temperature reaction of palladium(II)
chloride and an excess of gallium(III) chloride (molar ratio 1:4.4)
in toluene, red/brown crystals in moderate yield were acquired.
The single crystal structure determination proved the compound
to be [Pd2(Ga2Cl7)2(C7H8)2] (1).
In the search for suitable solvents for dissolving PHAs, naphtha-
lene, anthracene and biphenyl a number of solvents were tested.
Toluene as well as benzene, were tested but only the previously
known
[Pd2(GaCl4)2(C6H6)2],
[Pd2(Ga2Cl7)2(C6H6)2]
and
[Pd2(GaCl4)2(C7H8)2] compounds were formed, except for the
new [Pd2(Ga2Cl7)2(C7H8)2] (1). This indicates that these simple aro-
matic solvents outcompete or simply outnumber the three PAHs in
terms of coordination to the Pd22þ unit. From the reaction of PdCl2
and GaCl3 in a biphenyl-saturated mesitylene solution deeply red/
brownish crystals were collected. These crystals were of the gen-
eral composition [Pd2(GaCl4)2(C9H12)2]ꢀC9H12 (2). The solubility of
biphenyl and naphthalene in mesitylene, as well as in toluene
and benzene, was shown to be good. However, anthracene shows
a generally low solubility.
Chlorobenzene was also used as solvent for the reaction of PdCl2
and GaCl3 with naphthalene, anthracene and biphenyl, respec-
tively. Chlorobenzene was anticipated to be a good solvent for
the palladium reaction, because of its weak base character. It was
also expected to retard the oxidation (i.e. chlorination) of the arene
solvent and the following reduction of palladium(II) to palla-
dium(I), because of the already present chlorine substituent. Chlo-
robenzene showed similar solubility for naphthalene, biphenyl and
anthracene as mesitylene, toluene and benzene. The reaction be-
tween PdCl2 and GaCl3 in chlorobenzene resulted in brown/black
needle-shaped crystals of [Pd2(GaCl4)2(C6H5Cl)2] (3) within weeks.
It might well be that the reaction is slower, but it is evident that
chlorobenzene instead of the targeted PAHs coordinate to the
dipalladium unit. Fig. 1 shows the molecular structures of 1, 2
and 3. The results of the structural determinations are summarized
in Table 1.
Just as for the dipalladium–toluene compound, [Pd2(GaCl4)2-
(C7H8)2] earlier isolated in our group [4a], compound 1 displays a
central dipalladium unit enclosed by two sandwiching toluene
molecules. Each of the sandwiching toluenes shows a
g
5-coordina-
tion to the two palladium atoms of the dipalladium unit. As a result
of packing effects, the methyl groups of the two toluene molecules
are oriented in opposite direction, like a trans-arrangement. Each of
the palladium atoms in the dipalladium unit is bound to one of the
chloride atom of the Ga2Clꢁ anion forming the halogen bridge to
7
gallium.
In the formation of some arene coordinated f-block compounds
a deviation in the arene substituents out of the aromatic plane
have been observed [1c]. This phenomenon also occurs in struc-
tures 1, 2 and 3, see Table 3. An approximately 6.0° deviation in
Fig. 1. The molecular structure of 1 (a), 2 (b) and 3 (c).
the angle
a is observed for the methyl group of the toluene com-
pound (1). The chlorine atom of the chlorobenzene compound (3)
is tilting 8.4° out of the aromatic plane and the methyl groups of
the mesitylene compound (2) between 6.3° and 8.9°. The devia-
tions in angle of the substituting groups on the aromatic ring have
been noted, by Hubig et al. [10], to be correlated with the degree of
charge transfer from the arene donor to the coordinated acceptor.
In accordance with the discussion by Hubig et al. the deviation an-
gles measured; 6.0° (1), 6.3–8.9° (2) and 8.4° (3) indicate an
in the charge transfer from the arene donor to the coordinated
acceptor. The deviation angles measured for the three compounds
are however relatively small as to why they can still be expected to
exhibit high p-character.
The crystallographic data of 2 show a third, non-coordinated
mesitylene molecule at a distance of 3.9 Å from the aromatic plane
of the closest coordinating mesitylene molecule. This mesitylene is
interacting via
p-stacking (or p–p interaction), Fig. 1b. A similar
increasing
the arenes, opposed to the pure
pected for a compound with zero deviation angel. An increasing
-character with increasing deviation angle implies an increase
r
-character in the coordination of the metal unit to
structural feature, has previously been reported for the diplatinum
species [Pt2(GaCl4)2(C10H8)2]ꢀ2C6H6 containing naphthalene [4a].
This occurrence might be of a stabilizing nature or simply an effect
of packing in the crystal structure.
p
-character that would be ex-
r

J. Åkerstedt et al. / Journal of Organometallic Chemistry 695 (2010) 1513–1517
1515
Table 1
Crystallographic data and structure refinement of the X-ray studies of [Pd2(Ga2Cl7)2(C7H8)2] (1), [Pd2(GaCl4)2(C9H12)2]ꢀC9H12 (2) and [Pd2(Ga2Cl7)2(C6H5Cl)2] (3).
Compound
[Pd2(Ga2Cl7)2(C7H8)2]
[Pd2(GaCl4)2(C9H12)2]ꢀC9H12
[Pd2(Ga2Cl7)2(C6H5Cl)2]
Sum formula
Mr
C14H16Cl14Ga4Pd2
1172.25
C27H36Cl8Ga2Pd2
996.45
C12H10Cll6Ga4Pd2
1213.14
Lattice parameters
a (pm)
b (pm)
c (pm)
b (°)
916.06(9)
1887.0(3)
955.30(8)
97.615(8)
P21/n
1844.1(2)
1667.1(2)
2367.0(2)
915.08(13)
1844.85(14)
947.84(13)
99.00(1)
P21/n
Space group
Pbca
Cell volume (pm3)
1636.8(3) ꢂ 106
7277.1(12) ꢂ 106
1580.4(3) ꢂ 106
Z
4
8
2
Total number of reflections
Number of independent reflections, Rint
Number of L.S. parameters
16,937
3686, 0.041
155
35,967
6558, 0.084
361
17,850
3612, 0.046
154
R1
0.030
0.070
0.046
wR2
0.061
0.159
0.114
S
1.11
1.16
1.08
Residual electron density, peak/hole
0.50/ꢁ0.49
0.70/ꢁ0.66
1.92/ꢁ1.95
Table 3
The deviation angle of the substituent groups from the aromatic plane in 1–3.
Table 2
Palladium–palladium bond lengths in selected dipalladium-arene compounds.
Compound
Pd–Pd [Å]
Reference
[Pd2(GaCl4)2(C6H6)2]
[Pd2(Ga2Cl7)2(C6H6)2]
[Pd2(GaCl4)2(C7H8)2]
[Pd2(Ga2Cl7)2(C7H8)2]
[Pd2(GaCl4)2(C9H12)2]ꢀC9H12
[Pd2(Ga2Cl7)2(C6H5Cl)2]
2.584
2.562
2.572
2.561
2.538
2.567
[4a]
[4a]
[4a]
This work
This work
This work
Compound
a [°]
[Pd2(Ga2Cl7)2(C7H8)2]
6.0
[Pd2(Ga2Cl7)2(C6H5Cl)2]
[Pd2(GaCl4)2(C9H12)2]ꢀC9H12
8.4
6.3–8.9
The crystal determination of 3 is somewhat ambiguous, since
obtaining high-quality diffraction data proved difficult. The crys-
tallographic results indicate that coordination is not completely
uniform. A possible explanation to this observation might be that
the crystal structure includes a small amount of coordinated ben-
zene, as to solely chlorobenzene. The Pd–Pd distance in 3 is how-
ever almost the same as that of the corresponding benzene
compound, [Pd2(Ga2Cl7)2(C6H6)2] (2.562 Å), and the Pd–C distances
between 2.2 and 2.4 Å also shows good conformity with previous
results. The C–Cl distance in the chlorobenzene sandwiching li-
gands are 1.675 Å, which is in agreement with, the C–Cl distance
expected for chlorobenzene [11].
species is spectroscopically evident in solution, and that a strong
gallium(III)-mesitylene complex is formed between the two enti-
ties [12]. The formation of this relatively stable gallium(III)-mesit-
ylene complex might disfavor the formation of a mesitylene-
stabilized dipalladium compound. This can pose as an explanation
to why 2 has not been isolated from a simple mesitylene solution,
indicating the effect of dissolved biphenyl in the reaction. And in a
paradox way, the intended ligand sees itself outcompeted by the
solvent as ligand to Pd22þ; possibly through coordination to GaCl3/
Ga2Cl6, thus increasing mesitylene activity relative to the pure
GaCl3-mesitylene reaction medium.
The palladium–palladium distance in 1, 2.561 Å, is comparable
to that in the benzene analogue, but it is slightly longer than that
for the toluene analogue [Pd2(GaCl4)2(C7H6)2] (2.572 Å), as seen
in Table 2. The Pd–Pd distances indicate that the dipalladium units
are stabilized by direct metal–metal bonding. It also seems that the
slight difference in palladium–palladium distance between com-
Mesitylene, unlike benzene or toluene, has also been shown to
have a tendency of being strongly coordinating to the Pd22þ unit,
leading to disproportionation into Pd0 and PdII [4a]. However,
mesitylene has been reported to stabilize Hg22þ, in [Hg2(AlCl4)2-
(C9H12)2] [13], and it is in this work shown to stabilize Pd2þ in
2
[Pd2(GaCl4)2(C9H12)2]ꢀC9H12. It thus seems that there is a small
overall difference in the strength of the soft base donors mesity-
lene, toluene and benzene.
pounds with the Ga2Clꢁ and the GaClꢁ4 halogen bridging ligands
7
have negligible effects on the overall geometry of the compounds.
The Pd–C distances in 1 vary from 2.2 to 2.5 Å, which is comparable
to the palladium–carbon distances in [Pd2(GaCl4)2(C7H8)2].
The palladium–palladium distance in 2, 2.538 Å, is slightly
shorter than those of 1 and 3 and the other compounds in Table 2.
This shortening of the Pd–Pd bond might be induced by the coor-
In order to investigate the interactions between the different
arenes and the dipalladium unit, two simple theoretical models
were constructed; Pd2Cl2(arene)2 and Pd2(GaCl4)2(arene)2
(arene = benzene, toluene, mestiylene or chlorobenzene). Quantum
chemical calculations on these models at hybrid density functional
level show the Pd2Cl2(arene)2 model to be the most informative
model because of its simplicity and ease of interpretation. Using
the Pd2Cl2(arene)2 model the interaction energy between chloro-
benzene and the dipalladium unit in Pd2Cl2(chlorobenzene)2 was
calculated to be nearly three times that of benzene. The interaction
between chlorobenzene and the dipalladium unit in compound
Pd2(Ga2Cl7)2(C6H5Cl)2] might be of electrostatic nature. Toluene
on the other hand shows quite similar interaction energy to that
of benzene, while mesitylene is interacting slightly stronger, 1.5
times that of benzene. These relatively small differences in the
dinating mesitylenes. The mesitylenes in 2 each show a
g
4-coordi-
nation to the two palladium atoms of the dipalladium unit. The
mesitylenes are oriented in a parallel fashion, with the substituting
methyl groups in an eclipsed conformation. The Pd–C distances in
2 are between 2.2 and 2.4 Å. The reaction of PdCl2 with GaCl3 in
biphenyl-saturated mesitylene was carried out in an excess of
GaCl3 and resulted in the formation of 2 containing the GaCl4ꢁ
ligand.
It has previously been suggested from spectroscopic studies of
the GaCl3-mesitylene system that only the monomeric GaCl3

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J. Åkerstedt et al. / Journal of Organometallic Chemistry 695 (2010) 1513–1517
interaction energies calculated are in agreement with the small dif-
ferences in the observed deviation angles of the arene substituents
out of the aromatic plane for the different compounds. There thus
seems to be an small overall difference in the stability of the three
different compounds isolated. The Pd–Pd bond distances were also
calculated, 2.629–2.631 Å and are somewhat longer than those
measured for 1, 2 and 3. The calculated Pd–Pd bond distance along
with calculated Raman frequencies are shown in Table S1 in Sup-
plementary information.
4.1.1. Synthesis of [Pd2(Ga2Cl7)2(C7H8)2] (1)
Toluene (1 ml) was added to the solid mixture of PdCl2 (0.025 g,
0.141 mmol) and GaCl3 (0.11 g, 0.614 mmol). The mixture was
gently shaken and unreacted starting material was removed after
a few hours. The reaction flask was wrapped in aluminum foil
and left at room-temperature. Within days dark red crystals of 1
(yield ꢄ 69%) were formed.
4.1.2. Synthesis of [Pd2(GaCl4)2(C9H12)2]ꢀC9H12 (2)
The synthesis of 1, 2 and 3 were performed using an excess of
GaCl3. It was indicated by variations in concentration that an equi-
Compound 2 was prepared by the addition of a 1 ml saturated
solution of biphenyl in mesitylene to the solid mixture of PdCl2
(0.025 g, 0.141 mmol) and GaCl3 (0.11 g, 0.614 mmol). After a few
hours unreacted solid material was removed and the deep red
reaction mixture was wrapped in aluminum foil. Deeply red,
almost black crystals of 2 appeared within a few weeks (yield 21%).
librium of anions GaClꢁ4 and Ga2Clꢁ govern these reactions and that
7
which compound is formed is a matter of packing effects in the so-
lid compound formed. It was earlier suggested by Allegra et al. that
an excess of AlCl3, molar ratio 1:1.6 of PdCl2 to GaCl3, would gen-
erate [Pd2(Al2Cl7)(C6H6)2], while a 1:1 molar ratio would result in
[Pd2(AlCl4)(C6H6)2] [3]. In this work a 1:4.4 molar ratio of GaCl3
4.1.3. Synthesis of [Pd2(Ga2Cl7)2(C6H5Cl)2] (3)
leads to the formation of compounds with Ga2Clꢁ ligands for 1
7
The synthesis of 3 was performed in a similar manner to that of
1. Chlorobenzene (1 ml) was added to anhydrous solid PdCl2
(0.025 g, 0.141 mmol) and GaCl3 (0.11 g, 0.614 mmol). The reaction
vessel was left in an aluminum foil wrapped vessel for a few weeks,
after which a few red/brownish crystals had formed. The approxi-
mate yield for the reaction was merely 6%.
and 3 and GaClꢁ4 for 2.
Raman spectra of 1 in toluene show a band at 152 cmꢁ1 that
might be assigned to the symmetric Pd–Pd stretch. Quantum
chemical calculations on the toluene sandwich compound (1) show
this palladium–palladium vibration band at 159 cmꢁ1, Table S1 in
Supplementary information. The bands at 356 cmꢁ1 and 384 cmꢁ1
are assigned to chlorogallates(III), GaClꢁ, Ga2Clꢁ and Ga3Clꢁ10, see
4
7
4.2. Crystal structure determination
Supplementary information Fig. S1. The Raman spectra of the prod-
uct solution of [Pd2(GaCl4)2(C9H12)2]ꢀC9H12 (2) shows a band at
360 cmꢁ1 that can be assigned to the chlorogallate(III) GaClꢁ4 , see
Fig. S2 in Supplementary information. Because of the low solubility
of the crystals of 2 in mesitylene, it proved difficult to dissolve in
detectable amounts. Compound 3 decomposed during Raman
detection, wherefore no results from these measurements are
presented.
The isolated compounds experience sensitivity to moisture and
oxygen. Therefore all crystals were selected under a protecting
atmosphere, and inserted into glass capillaries prior to crystal
structure determination. Diffraction data was collected on a Bru-
ker-Nonius KappaCCD diffractometer. For 2, a linear decomposi-
tion correction was applied [14]. Structural models were
obtained using direct methods and H atoms were placed at calcu-
lated positions [15]. Table 1 summarises the details of the structure
determinations.
3. Conclusion
In summary, the syntheses performed resulted in the formation
of three new arene-stabilized binuclear palladium(I) compounds.
[Pd2(Ga2Cl7)2(C7H8)2] was synthesized from PdCl2 in a toluene
reaction mixture with GaCl3 in large excess. It was found that
mesitylene coordinates more easily to the dipalladium core than
biphenyl when mixed in the reaction with palladium(II) chloride
and gallium(III) chloride, a reaction which resulted in the forma-
tion of [Pd2(GaCl4)2(C9H12)2]ꢀC9H12. Moreover, the chlorobenzene
compound, [Pd2(Ga2Cl7)2(C6H5Cl)2], was isolated. The results from
calculations show chlorobenzene to be the strongest interacting
arene to the dipalladium unit, of the arenes used within this work.
4.3. Raman spectroscopy
Raman spectroscopic analyses on the products 1, 2 and 3 were
carried out on a BioRad FTS 6000 spectrometer. The Raman spec-
trometer was equipped with a Raman accessory, 2W Nd:YAG laser
(k = 1064 nm), a quartz beam splitter and a liquid nitrogen-cooled
solid state Ge detector. A resolution of 4.0 cmꢁ1 was used in all
measurements. The samples were studied in sealed standard
NMR-tubes at room-temperature.
4.4. Quantum chemical calculations
The calculations were made using GAUSSIAN 03 (rev. C02) [16].
Calculations were made using the hybrid density functional
B3PW91. Basis sets for H, C and Cl were of ‘6-311G’ quality supple-
mented with diffuse and polarization functions. Quasi-relativistic
effective core potentials (28 electrons, MWB) were used for Ga
and Pd together with contracted [4s4p]/(2s2p) and [8s7p6d]/
(6s5p3d) valence functions, respectively [17–19].
4. Experimental
4.1. Synthesis
All reactions were performed under an inert atmosphere of
deoxygenated and dried nitrogen in a glove box. The starting mate-
rials; GaCl3 (Sigma–Aldrich, anhydrous 99.99%), PdCl2 (Sigma–
Aldrich, 99.999%), naphthalene (Avocado, 98.0%), anthracene
(Avocado, 95.0%), biphenyl (Sigma–Aldrich, 99.5%) were used as re-
ceived. The solvents; toluene (Sigma–Aldrich, 99.9%), mesitylene
(Fluka, 98.0%) and chlorobenzene (Sigma–Aldrich, anhydrous,
99.8%) were dried over molecular sieves prior to use.
Acknowledgement
The Swedish Research Council (VR) is acknowledged for finan-
cial support.
The following general reaction scheme represents the synthetic
approach used for the synthesis of 1, 2 and 3:
Appendix A. Supplementary data
PdCl2 þ GaCl3 þ arene 9 ½PdI ðareneÞ L2ꢃ;
whereLꢁ ¼ GaCl4ꢁ or Ga2Clꢁ2
ð1Þ
2
CCDC 757219 (1), 757218 (2) and 757220 (3) contains the
supplementary crystallographic data for this paper. These data
7

J. Åkerstedt et al. / Journal of Organometallic Chemistry 695 (2010) 1513–1517
1517
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