Binuclear palladium(I) and platinum(I) dimers stabilized by aromatic ligands: Synthesis, structural characterization and reactivity with carbon monoxide

Article abstract of DOI:10.1016/S0020-1693(02)01577-3

Reaction of PdCl₂ with excess of GaCl₃ in aromatic solvents leads to binuclear compounds of the general formula [Pd ₂X₂(arene)₂], where arene=C₆H ₆, X^-=Ga₂Cl₇^- (1); arene=C₇H₈, X^-=GaCl₄^- (2). The solid-state structures of compounds 1 and 2 have been determined by X-ray crystallography. Two molecules of the arene are bound to the dipalladium unit. The compounds 1 and 2 do not react with triphenyl phosphine. Reaction of carbon monoxide with 1 in benzene solution yields [Pd₂(GaCl ₄)₂(C₆H₆)₂] (3), for which the crystal structure has also been determined. The compound [Pt ₂(GaCl₄)₂(C₁₀H₁₀) ₂]·2C₆H₆ (4), which was obtained by reaction of K₂[PtCl₄] with GaCl₃ and naphthalene in a benzene solution, has a similar structure in the solid state.

Full text of DOI:10.1016/S0020-1693(02)01577-3

Inorganica Chimica Acta 350 (2003) 449Á454
/
www.elsevier.com/locate/ica
Binuclear palladium(I) and platinum(I) dimers stabilized by aromatic
ligands: synthesis, structural characterization and reactivity with
carbon monoxide
Mikhail Gorlov a, Andreas Fischer b, Lars Kloo b,*
a Department of Inorganic Chemistry, St. Petersburg State Institute of Technology, 198013 Moskovsky pr., 26 St. Petersburg, Russia
b Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden
Received 8 July 2002; accepted 4 November 2002
Dedicated in honour of Professor Pierre Braunstein
Abstract
Reaction of PdCl2 with excess of GaCl3 in aromatic solvents leads to binuclear compounds of the general formula
[Pd2X2(arene)2], where areneꢀ
/
C6H6, Xꢁ ꢀ
/
Ga2Cl7ꢁ (1); areneꢀ
/
C7H8, Xꢁ ꢀGaCl4ꢁ (2). The solid-state structures of compounds
1 and 2 have been determined by X-ray crystallography. Two molecules of the arene are bound to the dipalladium unit. The
/
compounds 1 and 2 do not react with triphenyl phosphine. Reaction of carbon monoxide with 1 in benzene solution yields
[Pd2(GaCl4)2(C6H6)2] (3), for which the crystal structure has also been determined. The compound [Pt2(GaCl4)2(C10H10)2]×
/2C6H6
(4), which was obtained by reaction of K2[PtCl4] with GaCl3 and naphthalene in a benzene solution, has a similar structure in the
solid state.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; Platinum; Binuclear complexes; Gallium chloride; Carbon monoxide
1. Introduction
arenes [14]. The formal oxidation state of palladium in
these aggregates is ꢂ2. On the other hand, the binuclear
palladium(I) complexes [(h-arene)PdX]2 (where Xꢁ ꢀ
AlCl4ꢁ, Al2Cl7ꢁ; areneꢀ
benzene, toluene) were pre-
pared by reaction of PdCl2 with AlCl3 and Al in arene
solutions [15]. Crystal structures of the benzene com-
plexes were determined [15,16]. It was also observed that
these compounds are active catalysts for ethylene
dimerization [17].
/
Palladium complexes play an outstanding role in
/
organic synthesis as catalysts of hydrogenation [1Á
oxidation [4], carbonylation [5Á7], hydrosilylation [8],
arylation [9] of a wide range of organic compounds.
Another unique property of palladium complexes is
their ability to display allylic coordination, and some
palladium complexes have been used as catalysts in
allylic substitution reactions [10,11].
/3],
/
/
It was found in our lab [18] that low-valent di- and
trimercury cations are stable in benzene solution of
GaCl3. It was proposed that this may be the conse-
quence of a soft-base stabilization provided by the
interaction between the aromatic molecules and sub-
valent cations.
In the last years, Heaton and co-workers reported
that homoleptic dinuclear carbonyl platinum cluster
[{Pt(CO)3}2]2ꢂ can be prepared in strongly acidic
media, such as concentrated sulfuric acid [19]. The
low-valent Group 10 homoleptic carbonyls have turned
The common oxidation states of palladium in orga-
nometallic chemistry are 0 and ꢂ2. It was proposed that
/
palladium(I) complexes are intermediates in a number of
reactions [12], but until 1971 only two palladium(I)
compounds had been obtained [13].
Recently, Olmstead and co-workers have found that
Pd6Cl12 forms stable supramolecular aggregates with
* Corresponding author.
E-mail address: larsa@inorg.kth.se (L. Kloo).
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01577-3

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M. Gorlov et al. / Inorganica Chimica Acta 350 (2003) 449Á454
/
out to be excellent catalysts for the carbonylation of
olefins [20Á22].
or Ga2Cl7ꢁ; areneꢀ
C6H6, C7H8. The predominant
/
reducing agent with respect to the added PdII is the
arene.
/
The main objective of this work is the stabilization of
subvalent cations of the platinum metals by gallium
trichloride dissolved in aromatic solvents, and the
subsequent investigation of the reactivity of these
compounds with respect to carbon monoxide.
This article reports the syntheses and crystal struc-
tures of four new binuclear palladium(I) and platinum(I)
complexes containing GaCl4ꢁ or Ga2Cl7ꢁ groups
stabilized by aromatic molecules, together with their
reactivity versus carbon monoxide and triphenyl phos-
phine.
The products crystallize as dark red, almost black
solids. The solid-state structures of the compounds
[Pd2(Ga2Cl7)2(C6H6)2] (1), [Pd2(GaCl4)2(C7H8)2] (2)
and [Pd2(GaCl4)2(C6H6)2] (3) have been determined by
single crystal X-ray diffraction (Table 1).
The compounds 1Á
3 contain a Pd22ꢂ unit as the
/
central structural feature. The short distance between
palladium atoms strongly suggests that the Pd22ꢂ unit is
stabilized by metalÁmetal bonding. The dipalladium
/
unit is coordinated by two parallel arene molecules in a
sandwich configuration (Fig. 1). The coordination
sphere is completed by Ga2Cl7ꢁ (1) and GaCl4ꢁ (2
and 3) anions, which are bound to the Pd atoms via one
chlorine atom. In all compounds described in this work,
this complex resides on an inversion center, thus yielding
a symmetric arrangement whose halves are crystal-
lographically equivalent. The type of ligand has hardly
2. Results and discussion
2.1. Palladium complexes
The reaction of PdCl2 with an excess of GaCl3 in
aromatic solvents yields binuclear compounds of the
general formula [Pd2X2(arene)2], where Xꢁ ꢀGaCl4ꢁ
any influence on the geometry of the complex: the Pdꢀ
/
/
Pd distance in the toluene complex is 257.2 pm and thus
Table 1
Crystallographic data and experimental details of the X-ray studies of [Pd2(Ga2Cl7)2(C6H6)2] (1), [Pd2(GaCl4)2(C7H8)2] (2), [Pd2(GaCl4)2(C6H6)2] (3)
and [Pt2(GaCl4)2(C10H10)2]×
/
2C6H6 (4)
Compound
1
2
3
4
Sum formula
Cell constants
C6H6Cl7Ga2Pd
C7H8Cl4GaPd
C6H6Cl4GaPd
C16H16Cl4GaPt
aꢀ
bꢀ
cꢀ
aꢀ
bꢀ
gꢀ
755.53(3)ꢃ
/
925.50(2) pm
aꢀ
bꢀ
cꢀ
/
767.19(2) pm
aꢀ
bꢀ
cꢀ
/
1397.42(2) pm
aꢀ
bꢀ
cꢀ
/
2386.95(2) pm
987.67(1) pm
1964.79(2) pm
/
939.70(3) pm
/1670.76(5) pm
/1107.29(2) pm
/
/
946.50(2) pm
/
930.83(3) pm
91.017(1)8
1192.94(6)ꢃ
106
/1444.58(3)
/
/83.807(1)8
/70.294(1)8
bꢀ
/
bꢀ
/
101.9312(7)8
106
bꢀ/96.0285(3)8
/77.317(1)8
Cell volume (pm3)
/
106
/
2186.98(7)ꢃ
/
4606.41(8)ꢃ
/
106
Z
2
2.515
4
2.283
8
2.406
8
1.993
r(calc.) (g cmꢁ3
Crystal system, space group triclinic, P1
)
monoclinic, P21/c
297
monoclinic, C2/c
297
monoclinic, P21/c
120
Temperature (K)
Radiation
Absorption coefficient m
100
Mo Ka, lꢀ
/
71.073 pm
50.3
59.2
46.1
77.1
(cmꢁ1
)
Absorption correction
numerical
u Range
4.218B
/
u B/27.458
4.198B
/
u B/27.488
4.568B/u B/27.498
4.138B
/
u B27.478
/
Number of measured reflec- 17 021
tions
13 614
13 778
41 121
Number of unique reflec-
tions
Rint (%)
3436
2700
2487
10 422
3.31
163
6.14
118
5.70
109
13.1
420
Number of refined para-
meters
Ratio reflections/parameters 21
23
23
25
Residuals
wR2ꢀ
/
4.93% (all reflections)
wR2ꢀ
tions)
R1ꢀ5.15% (all reflec-
tions)
R1ꢀ3.56% (2169 ob-
served)
0.44/ꢁ0.50
/
8.21% (all reflec-
wR2ꢀ
tions)
R1ꢀ7.59% (all reflec-
tions)
R1ꢀ7.30% (2235 ob-
served)
0.92/ꢁ1.00
/
7.59% (all reflec-
wR2ꢀ23.7% (all reflec-
tions)
/
R1ꢀ
/
2.32% (all reflections)
/
/
R1ꢀ
tions)
R1ꢀ8.34% (9056 ob-
served)
4.34/ꢁ4.89
/9.25% (all reflec-
R1ꢀ
/
2.09% (3222 observed re-
/
/
/
flections)
0.61/ꢁ0.62
Difference electron density
/
/
/
/

M. Gorlov et al. / Inorganica Chimica Acta 350 (2003) 449Á
/454
451
Fig. 2. Orientation of benzene ligands in 3 (A) and 1 (B).
It is important to note that PdCl2 reacts with GaCl3 in
ethyl benzene or mesitylene, but instead of the forma-
tion of dipalladium cluster metallic palladium is ob-
served. It seems likely that the aromatic molecules are
important in stabilizing the Pd22ꢂ units through soft-
base interactions [18]. However, just as for the subvalent
Hgm2ꢂ cations, the stabilization only appears for rela-
tively weak soft bases, e.g. benzene and toluene.
Stronger bases tend to stabilize the PdII state instead,
Fig. 1. View of 1 (A), 3 (B) and 2 (C).
lies in between those in the two benzene complexes in
which it is 256.2 (1) and 258.4 (3). The shortest Pdꢀ
/
C
leading to a disproportionation reaction Pd22ꢂ 0
/
Pd0(s)ꢂ
/
distance in this complex is 221.6 pm and is thus the same
as in the benzene complexes (221.0 pm in 1 and 222.1 in
3).
PdII.
On the other hand, complex formation using 1,2,4-
trichlorobenzene as solvent is not observed. This
strongly indicates that the predominant reduction of
PdII occurs via oxidation (i.e. chlorination) of the arenes
giving products such as chlorobenzenes. The use of an
already chlorinated solvent consequently retards any
reduction of PdII. We can, therefore, conclude that these
compounds are extremely sensitive to small changes in
electronic and steric effects caused by the arenes.
The reaction between PdCl2, GaCl3 and ferrocene in
THF solution was also studied. The products of this
reaction are metallic palladium and blue crystals (Eq.
(1)). The X-ray analysis of the latter has shown that it is
[(C5H5)2Fe][GaCl4], whose crystal structure was recently
determined by Scholz and co-workers [23].
The synthesis and structure of the similar aluminium
compounds,
[Pd2(AlCl4)2(C6H6)2]
and
[Pd2(Al2Cl7)2(C6H6)2], were described by Allegra and
co-workers [15]. These compounds were prepared by
reaction of PdCl2 with AlCl3 and Al in benzene
solutions. The first compound was obtained using a
1:1 molar ratio of PdCl2 and AlCl3 (Pdꢀ
one with a molar ratio of 1:1.6. In the case of 2 in this
work, the corresponding PdꢀGa ratio was 1:4.5, but the
/
Al), the second
/
resulting complex, [Pd2(GaCl4)2(C7H8)2], contains only
GaCl4ꢁ groups. It seems that an excess of gallium
trichloride does not give compounds with Ga2Cl7ꢁ
groups.
Compounds 1 and 3 are isotypic with the respective
aluminum compounds published earlier [15,16]. The
PdCl2 ꢂ2GaCl3 ꢂ2(C5H5)2Fe
Pdꢀ
/
Pd distance in these complexes agrees with the
0 Pd0 ꢂ2[(C5H5)2Fe][GaCl4]
(1)
(s)
distances published previously. It is interesting to note,
that the benzene rings in 1 showed severe rotational
disorder at room temperature, which made a data
collection at 100 K necessary. The structure of 3, while
containing the same arene ligand, does not exhibit this
2.2. Platinum complexes
phenomenon. Additionally, the coordination of the Pdꢀ
Pd dumbbell is slightly different: in 3 a two-fold axis of
the benzene rings are exactly aligned with the PdꢀPd
bond, whereas they are slightly tilted in 1 (Fig. 2).
/
PtCl2 does not react with GaCl3 in benzene, toluene
or mesitylene even under heating. However, K2[PtCl4]
readily reacts with GaCl3 in these aromatic solvents to
give a dark brown solution. After several minutes the
/

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M. Gorlov et al. / Inorganica Chimica Acta 350 (2003) 449Á454
/
solution separates into two phases. The upper layer is
yellow while the lower one is dark brown. 195Pt NMR
spectra of upper layer (benzene system) show the
absence of platinum compounds in any detectable
amounts. The existence of liquid phase separation
have previously been observed in systems with mono-
nuclear mercury compounds Hg(arene)2(MCl4)2 where
MꢀAl, Ga [24], and in the saturated solution of Hg2Cl2
/
and GaCl3 in benzene [18].
Reaction of K2[PtCl4] with GaCl3 and naphthalene in
benzene solution yields [Pt2(GaCl4)2(C10H10)2]×
/2C6H6
(4) and KGaCl4 [25]. The crystal structure of 4 has been
determined (Table 1, Fig. 3).
The structure of 4 deviates significantly from those
presented so far. It consists of a central Pt2(GaCl4)2 unit
which is coordinated by two molecules of naphthalene.
In this structure, two crystallographically independent
complexes are present.
Additionally, the structure contains two molecules of
benzene per complex, which probably serve as ‘separa-
tors’ between the rather bulky naphthalene ligands (Fig.
4). The Ptꢀ
/
Pt distances are 254.4 and 254.2 pm,
respectively. The shortest Ptꢀ
/C distances in these
complexes are 217.2 pm in both complexes.
Fig. 4. Packing diagram of 4.
contrast to the initial solution, which is stable with
respect to disproportionation.
2.3. Reactivity towards PPh3 and CO
Carbon monoxide was bubbled through the solution
of PdCl2 and GaCl3 in toluene for 4 h, but only
compound 2 was isolated in the solid state.
All attempts to substitute any of the ligands in 1 and 2
by triphenyl phosphine (PPh3) were unsuccessful. Even
in the presence of excess of PPh3, PdCl2 and GaCl3 in
benzene or toluene yield 1 and 2, respectively.
The reaction of 1 and 2 with carbon monoxide was
also studied. After bubbling CO gas through the
benzene solution of 1 for 2 h red crystals formed after
a delay. The solid-state crystal structure of this com-
pound was unexpected. It was found that the compound
is the binuclear cluster, [Pd2(GaCl4)2(C6H6)2] (3), and it
does not contain any CO ligands (Table 1, Fig. 1 (B)).
After exposure to CO the solution becomes sensitive to
air with the formation of metallic palladium. This is in
Bubbling of CO through the arene (benzene, toluene,
mesitylene) solutions of K2[PtCl4] and GaCl3 and
subsequent evaporation led to a red or brown oil and
KGaCl4. A comparison of the 195Pt NMR spectra of the
solutions before and after CO exposure shows that CO
reacts with the platinum compounds. There is one peak
at ꢁ
solution of K2[PtCl4] and GaCl3. After bubbling of CO
through this solution the peak shifts to ꢁ293 ppm. No
/
137 ppm in the 195Pt NMR spectrum of benzene
/
detailed information could give clues to the species
present in solution. 13C NMR spectra were recorded but
they proved to be inconclusive; indicating a mixture of
products.
3. Conclusions
The complexes described above represent a develop-
ment of the chemistry of the organometallic soft-base
stabilized binuclear palladium(I) and platinum(I) com-
plexes. The compound [Pt2(GaCl4)2(C10H10)2]×
/
2C6H6 is
the first example of a platinum complex of this type. It
was found that the [Pd2X2(arene)2] compounds are
stable with respect to substitution with PPh3 or CO.
Fig. 3. View of 4.

M. Gorlov et al. / Inorganica Chimica Acta 350 (2003) 449Á
/454
453
4. Experimental
crystals were grown within 24 h at room temperature
(r.t.). Yield: 110.9 mg, 96%.
4.1. Starting materials
4.4.3. Synthesis of [Pd2(GaCl4)2(C6H6)2] (3)
The compound 1 (0.050 g, 0.044 mmol) was dissolved
in benzene (20 ml) under heating. The carbon monoxide
was bubbled through the solution for 2 h at r.t. The
color of solution turned from red to brown. The
solution was stored without cap and set aside for 1
week. Red crystals formed.
GaCl3 (Aldrich Chemical Company, 99.99%, H2OB
100 ppm) and PdCl2 (Aldrich Chemical Company,
/
99.9ꢂ%) were used as received. K2[PtCl4] was obtained
/
according to Ref. [26]. All solvents were dried prior to
use. Carbon monoxide 99.9995% quality was used.
4.2. NMR spectroscopy
4.4.4. Synthesis of [Pt2(GaCl4)2(C10H10)2]×
/
2C6H6 (4)
NMR spectra were obtained on a Bruker AMX 500
spectrometer. The spectra were recorded at 300 K in the
appropriate arene solution. The 195Pt NMR spectra
were recorded at 107.1 MHz and referenced versus an
aqueous solution of K2[PtCl4]. The 13C NMR spectra
were recorded at 125.7 MHz.
Benzene (2 ml) was added to the solid mixture of
anhydrous GaCl3 (0.043 g, 0.240 mmol), K2[PtCl4]
(0.050 g, 0.120 mmol), and naphthalene (0.061 g, 0.480
mmol). The mixture was stirred for 15 min. The dark-
brown solution was set aside for 2Á
plates formed and were washed twice by benzene. Yield:
1%. Since both the yield and reproducibility of this
/3 weeks. Red crystal
B
/
4.3. Structure determinations
compound were extremely low, elemental analysis could
not be carried out.
Due to the extreme sensitivity towards humidity,
crystals of all compounds were sealed inside glass
capillaries in a nitrogen atmosphere. Data were collected
on a Bruker Nonius KappaCCD diffractometer. Nu-
merical absorption corrections were applied [27]. The
positions of the Pd (Pt), Ga and Cl atoms could be
determined using Direct Methods (SHELXS-97). The
positions of the C atoms were determined in subsequent
difference Fourier syntheses. H atoms were placed at
calculated positions. All parameters were refined against
F2 using full-matrix least-squares (SHELXL-97). Further
details are to be found in Table 1.
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC Nos. 195958, 195959, 195960 and
195961 for compounds 1Á4, respectively. Copies of this
/
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: ꢂ44-1223-336-033; e-mail: depos-
/
it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.a-
c.uk).
4.4. Synthesis
All experiments were performed in a glove box under
an inert atmosphere of deoxygenated and dried nitrogen
Acknowledgements
(B1 ppm H2O).
/
We thank Dr. Z. Szabo for recording of NMR
spectra. The Swedish Institute is acknowledged for the
grant of The New Visby Programme to M.G., and the
Swedish National Research Council for financial sup-
port and funding of the X-ray diffractometer.
Elemental analyses of 1 and 2 show very small
abundance of carbon (1.2% for 1). The results most
likely are caused by decomposition through evaporation
of the arene, which is readily lost on storage (in contrast
to the immediate crystallographic analysis performed).
4.4.1. Synthesis of [Pd2(Ga2Cl7)2(C6H6)2] (1)
Benzene (2 ml) was added to the solid mixture of
anhydrous GaCl3 (0.441 g, 2.505 mmol), and PdCl2
(0.062 g, 0.352 mmol). The reaction flask was wrapped
in aluminium foil and set aside for 24 h. Dark red
crystals formed; yield 90.4 mg, 45%.
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