Dinuclear palladium(II) compounds with bridging cyclometalated phosphines. Synthesis, crystal structure and electrochemical study

Article abstract of DOI:10.1039/b609829k

The structural characterization of bis-cyclometalated palladium(ii) compounds of formula Pd₂[(-(C₆X₄)PPh ₂]₂(-O₂CR)₂ [X = H, R = CH ₃ (3), CF₃ (4), C(CH₃)₃ (5) and C₆F₅ (6); X = F, R = CH₃ (7) and CF ₃(8)], has confirmed its paddle wheel structure with two palladium atoms bridged by two acetates and two metalated phosphines in a head-to-tail arrangement. The Pd...Pd distances are in the range 2.6779(16)-2.7229(8) A. Under cyclic voltammetric conditions, compounds 3-6, in CH ₂Cl₂ solution, were found to undergo a reversible oxidation peak in the range of potential values 0.84-1.25 V. A second partially-reversible oxidation is observed at more positive potentials (1.37-1.55 V). For compounds 3-5 in the presence of chlorides, the first oxidation becomes a two-electron process presumably leading to a neutral [Pd(iii)-Pd(iii)] species with a metal-metal bond. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b609829k

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Dinuclear palladium(II) compounds with bridging cyclometalated phosphines.
Synthesis, crystal structure and electrochemical study†
Igor O. Koshevoy,a Pascual Lahuerta,*a Mercedes Sanau´,a M. Angeles Ubeda*a and Antonio Dome´nechb
Received 10th July 2006, Accepted 21st September 2006
First published as an Advance Article on the web 3rd October 2006
DOI: 10.1039/b609829k
The structural characterization of bis-cyclometalated palladium(II) compounds of formula
Pd2[(l-(C6X4)PPh2]2(l-O2CR)2 [X = H, R = CH3 (3), CF3 (4), C(CH3)3 (5) and C6F5 (6); X = F, R =
CH3 (7) and CF3(8)], has confirmed its paddle wheel structure with two palladium atoms bridged by
two acetates and two metalated phosphines in a head-to-tail arrangement. The Pd · · · Pd distances are
˚
in the range 2.6779(16)–2.7229(8) A. Under cyclic voltammetric conditions, compounds 3–6, in CH2Cl2
solution, were found to undergo a reversible oxidation peak in the range of potential values
0.84–1.25 V. A second partially-reversible oxidation is observed at more positive potentials (1.37–1.55
V). For compounds 3–5 in the presence of chlorides, the first oxidation becomes a two-electron process
presumably leading to a neutral [Pd(III)–Pd(III)] species with a metal–metal bond.
Introduction
Palladium complexes have been extensively used in catalysis.1–5
The chemistry of palladium is dominated by the oxidation state
II and monometallic compounds are the most frequent for this
metal in this particular oxidation state. There is a relatively large
family of palladium compounds that are bimetallic and have two
Scheme 1
palladium atoms at a variable distance. The most representative
contain two bridging ligands, frequently carboxylate ligands and
two cyclometalated ligands in a chelating coordination mode
(type A in Scheme 1).6 The shortest reported distances between
solution of species resulting from one or two electron oxidation
processes.30,31
7, 8
˚
palladium atoms in compounds of this type are 2.831 A.
However, the number of dinuclear palladium compounds with
bridging metalated ligands is considerably smaller. Palladium
compounds with bridging O–C,9–13 N–C14–19 or S–C ligands9,20
have been reported. In most of these compounds the palladium
atoms are quite separated and synergic interaction between the
two metal centers might only be possible in one case,20 with three
bridging ligands. Palladium complexes with ligands spanning a
P–C bridging coordination mode are usually restricted to those
ligands with the carbon atom being part of the p system: olefin,21
acetylene22 or arene.23 The typical paddle wheel structure (type B
in Scheme 1), common for other transition metals, is only observed
in a limited number of palladium(II) compounds.24–31
We have previously reported the synthesis and characterization
of palladium complexes of formula {Pd[l-(C6H4)PPh2]( l-Br)}4,
1, from the reaction of Pd(dba)2 with (o-BrC6H4)PPh2, being
the first example of a palladium compound with metalated
triphenylphosphine acting as a bidentate P–C donor ligand.32
Further studies confirmed the ability of the ligand [(C6H4)PPh2]−
to have bridging (l-) or chelating (j2-) coordination modes,
depending on the auxiliary ligands present in the complex.33 Bulky
2
phosphines stabilize mononuclear species of formula {Pd[g -
(C6H4)PPh2]Br[P]}, with a four-atom metallocycle, while small
phosphines give dinuclear compounds.
Preliminary reactions allowed us to obtain compounds of
formula Pd2[l-(C6H4)PPh2]2(l-O2CR)2 (R = CH3 (3) and CF3 (4))
that were not structurally characterized.
It is remarkable that very little information is available on
palladium(III) complexes, though electrochemical investigations
on dinuclear palladium(II) complexes confirmed the generation in
We report here the preparation of a new tetranuclear pal-
ladium(II) complex of formula {Pd[l-(C6F4)PPh2](l-Br)}4, (2)
and the synthesis and characterization of compounds Pd2[l-
(C6X4)PPh2]2(l-O2CR)2 (type C in Scheme 1) [X = H, R =
CF3 (4), C(CH3)3 (5) and C6F5 (6); X = F, R = CH3 (7) and
CF3 (8)]. Compounds 3–6 exhibit two reversible oxidation peaks
by cyclic voltammetry, being potential starting products in the
synthesis of dinuclear palladium compounds in higher oxidation
states. In future reports we shall describe the application of these
compounds in the synthesis of self assembled molecules containing
Pd2[l-(C6H4)PPh2]2 building blocks.28
aDepartamento de Qu´ımica Inorga´nica, Universitat de Valencia, Dr. Moliner
50, 46100, Burjassot, Valencia, Spain. E-mail: pascual.lahuerta@uv.es;
Fax: +34963544322; Tel: +34963543147
bDepartamento de Qu´ımica Anal´ıtica, Universitat de Valencia, Dr. Moliner
50, 46100, Burjassot, Valencia, Spain
† Electronic supplementary information (ESI) available: ORTEP diagrams
for the compounds (Fig. S1–S3); cyclic voltammograms (Fig. S4–S10); 31
P
NMR spectra (Fig. S11) and bond distances and angles for compounds
4–8 (Table S1). See DOI: 10.1039/b609829k
5536 | Dalton Trans., 2006, 5536–5541
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Synthesis of Pd2[l-(C6H4)PPh2]2(l-O2CR)2. R = CH3 (3), CF3
Experimental
(4), C(CH3)3 (5) and C6F5 (6)
General comments
Compounds 3 and 4 were prepared by the reported method.33
The other two compounds were prepared by reaction of 1
and stoichiometric amounts of the corresponding silver salt in
dichloromethane. After filtration, dilution with hexanes and evap-
oration of the solvents, 5 and 6 were obtained as yellow crystalline
solids (yield 85–95%). 5. Anal. Calc. for C46H46O4P2Pd2: C, 58.9;
H, 4.9. Found: C, 58.5; H, 5.0%. dP: 17.4 (s). dH: 7.71 (m, 2H), 7.41
(m, 3H), 7.29 (m, 1H), 7.20 (m, 1H), 7.00 (m, 2H), 6.80 (m, 3H),
6.58 (m, 2H), 0.54 (s, 9H). 6. Anal. Calc. for C50H28F10O4P2Pd2: C,
51.9; H, 2.4. Found: C, 51.9; H, 2.5%. dP: 19.4 (s). dH: 6.6–7.7 (m,
phenyl protons). Single crystals of these complexes, suitable for
X-ray diffraction, were grown at room temperature by diffusion
of heptane into their chloroform solutions.
34
35
The starting compounds Pd(dba)2 and P(o-BrC6F4)Ph2 were
prepared according to literature procedures. Reagent grade sol-
vents, dichloromethane, hexane, toluene, diethyl ether, acetonitrile
were used without purification. The solution H and 31P NMR
1
spectra were recorded from CDCl3 solutions on Bruker Avance 300
and Avance 400 spectrometers. The chemical shifts were referenced
to residual solvent resonance and external 85% H3PO4 in the
1H and 31P spectra, respectively. Microanalyses were carried
out at the Analytical Laboratory of the University of Joensuu,
Finland. Column chromatography was performed on silica, 35–
70 lm. The electrochemical measurements were performed in
CH2Cl2 at a scan rate of 0.05 V s−1, using tetrabutylammonium
hexafluorophosphate (Fluka) (0.1 M) as supporting electrolyte.
CCDC reference numbers 612060–612064. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b609829k
X-Ray
crystal
structure
data
for
compound
4.
=
Synthesis of {Pd[l-(C6F4)PPh2](l-Br)}4 (2). Method (a):
Pd(dba)2 (300 mg, 0.522 mmol) and toluene (1.5 mL) were put
into a Schlenk tube and were degassed. P(o-BrC6F4)Ph2 (210 mg,
0.508 mmol) was added in one portion in a flow of argon
and the reaction mixture was stirred at room temperature for
1 h. Then, the temperature was gradually increased to 90 ◦C
and stirring was continued for 15 h. After that, a fine greenish
precipitate and a dark brown solution were formed. The reaction
mixture was allowed to reach room temperature and the precipitate
was isolated by filtration, washed with toluene (2 × 4 mL),
diethyl ether (2 × 4 mL) and vacuum dried (205 mg). From
this solid, 135 mg (52%) of compound 2 can be extracted by
refluxing in dichloromethane, filtering off the insoluble material
and crystallization from dichloromethane–methanol mixtures.
Due to the low solubility of compound 2 this purification process
is very time consuming. Fortunately, the crude compound 2 can
be used for the synthesis of carboxylate derivatives 7 and 8 with
moderate yields (see later).
Method (b): compound 2 was readily obtained from the reaction
of compound 7 with excess of tetraethylammonium bromide
in an acetone–methanol mixture. In these conditions a yellow
precipitate was slowly formed. The filtered solid was washed
with methanol, acetone, diethyl ether and dried under vacuum
to give 2 (93%) as a yellow crystalline solid. Anal. Calc. for
C72H40Br4F16P4Pd4: C, 41.6; H, 1.9. Found: C, 41.5; H, 2.0%. dP
27.0 (m).
C40H28F6O4P2Pd2, monoclinic, space group P2(1)/n,◦
a
˚
12.315(3), b = 21.427(6), c = 14.530(4) A, b = 94.822(7) , V =
3820.6(17) A , Z = 4, qcalcd = 1.671 g cm−3, crystal dimensions:
3
˚
0.30 × 0.24 × 0.22 mm3; Bruker P4 diffractometer; Mo Ka
radiation, 153(2) K; 32 408 reflections, 11 655 independent; (l =
1.095 mm−1); refinement (on F2) with SHELXTL (version 6.1),
487 parameters, 0 restraints, R1 = 0.0479 (I > 2r) and wR2 (all
data) = 0.1197, GOF = 1.007, max/min residual electron density:
−3
˚
0.987/−0.767 e A .
X-Ray crystal structure data for compound 5. C46H46O4P2Pd2,
orthorhombic, space group Pccn, a = 10.9390(3), b = 15.9320(4),
3
˚
˚
c = 23.5800(6) A, V = 4109.53(19) A , Z = 4, qcalcd
=
1.515 g cm−3, crystal dimensions: 0.28 × 0.25 × 0.23 mm3;
Kappa CCD diffractometer; Mo Ka radiation, 293(2) K; 20
460 reflections, 4694 independent; (l = 0.996 mm−1); refine-
ment (on F2) with SHELXTL (version 6.1), 248 parameters,
0 restraints, R1 = 0.0957 (I > 2r) and wR2 (all data) =
0.3302, GOF = 1.047, max/min residual electron density:
−3
˚
1.877/−1.819 e A .
X-Ray
crystal
structure
data
for
compound
6.
C50H28F10O4P2Pd2, triclinic, space group P-1, a = 14.56300(10),
˚
b = 14.9110(2), c = 23.1350(3) A, a = 95.5460(5), b = 94.822(7),
c = 113.1221(5), V = 4541.94(9) A , Z = 4, qcalcd = 1.693 g cm−3,
3
˚
crystal dimensions: 0.24 × 0.23 × 0.23 mm3; Kappa CCD
diffractometer; Mo Ka radiation, 293(2) K; 47 438 reflections,
20 179 independent; (l = 0.949 mm−1); refinement (on F2) with
SHELXTL (version 6.1), 1225 parameters, 0 restraints, R1
=
0.0614 (I > 2r) and wR2 (all data) = 0.2005, GOF = 1.017,
Synthesis of the silver carboxylate salts
−3
˚
max/min residual electron density: 0.945/−1.129 e A .
All the remaining compounds were prepared, reacting compounds
1 or 2 with the corresponding silver salt. Silver acetate or
trifluoroacetate were commercially available. The remaining silver
salts were prepared as follows. A solution of AgNO3 (850 mg,
5 mmol) in water (3 mL) was added, under vigorous stirring, to
a solution of HO2CR (R = C(CH3)3, C6F5) (5 mmol) in 5 mL of
1M ammonium hydroxide solution. After filtration, AgO2CR was
obtained as a white precipitate, that was washed with water (2 ×
4 mL) and vacuum dried (70–80%). It was used without further
purification.
Synthesis of Pd2[l-(C6F4)PPh2]2(l-O2CR)2. R = CH3 (7), CF3 (8)
Crude 2 (150 mg) was suspended in a CH2Cl2–HO2CR (30 : 1 v/v)
mixture (30 mL) and excess of AgO2CR was added; the mixture
was gently refluxed in the absence of light for 24 h. The resulting
dark suspension was cooled at room temperature, diluted with
diethyl ether (5 mL) and passed through a silica column (1.5 ×
6 cm). The solvents were removed under vacuum to give an orange
residue that was extracted with diethyl ether. After addition of
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hexanes and partially removing the solvent under vacuum 7(62 mg,
43%) and 8(67 mg, 42%) were obtained as yellow crystalline solids.
7: (Anal. Calc. for C40H26F8O4P2Pd2: C, 48.2; H, 2.63. Found: C,
48.3; H, 3.02%. dP: 15.5 (m). dH: 7.78 (m, 2H), 7.55 (m, 3H), 7.44
(m, 1H), 7.23 (m, 2H), 7.00 (m, 2H), 1.19 (s, 3H). 8: Anal. Calc.
for C40H20F14O4P2Pd2 : C, 43.4; H, 1.8. Found: C, 43.2; H, 1.9%.
dP: 18.0 (m). dH: 6.6–7.7 (m, phenyl protons).
Single crystals suitable for X-ray diffraction study were grown,
at room temperature, by slow evaporation of a hexane solution of
7 and by diffusion of heptane into a chloroform solution of 8.
couple was +0.55 V. Prior to each run the working electrode was
polished during 60 s with a BAS polishing cloth.
Results and discussion
Following our previous investigations32,33 we have reacted the
tetranuclear compound {Pd[l-(C6H4)PPh2](l-Br)}4, 1, with differ-
ent silver carboxylates to form compounds that, based on spectro-
scopic data, were tentatively described as dinuclear compounds of
formula Pd2[l-(C6H4)PPh2]2(l-O2CR)2, R = C(CH3)3 (5) and C6F5
(6). These compounds were isolated in high yield as crystalline
materials (Scheme 2).
X-Ray
crystal
structure
data
for
compound
7.
C40H26F8O4P2Pd2, monoclinic, space group C(2)/c,
a
=
The new tetranuclear compound 2 was also prepared from
the reaction of Pd(dba)2 (dba = dibenzylideneacetone) with P(o-
BrC6F4)Ph2, according to the synthetic procedure described for
1,32 but in considerably lower yield. After careful modification of
the reaction conditions, 2 could be isolated with a maximum yield
of 40%. However, the purification of this compound became long
and tedious, due to its very low solubility that made it difficult to
separate from insoluble impurities also formed. The best solution
was to treat the reaction mixture with a silver carboxylate; only 2
reacted forming the corresponding compounds of formula Pd2[l-
(C6F4)PPh2]2(l-O2CR)2, [R = CH3 (7), CF3 (8)] that were soluble
and could be easily crystallized. Pure compound 2 could be readily
prepared by reacting either of these two carboxylate compounds
with Et4N+Br−.
◦
˚
22.0070(9), b = 9.7540(4), c = 20.5150(11) A, b = 106.3300(18) ,
3
V = 4226.0(3) A , Z = 4, qcalcd = 1.568 g cm−3, crystal dimensions:
˚
0.30 × 0.27 × 0.23 mm3; Kappa CCD diffractometer; Mo Ka
radiation, 293(2) K; 13 617 reflections, 4604 independent; (l =
0.999 mm−1); refinement (on F2) with SHELXTL (version 6.1),
255 parameters, 0 restraints, R1 = 0.0856 (I > 2r) and wR2 (all
data) = 0.2757, GOF = 1.044, max/min residual electron density:
−3
˚
1.569/−0.857 e A .
X-Ray
crystal
structure
data
for
compound
8.
C40H20F14O4P2Pd2, orthorhombic, space group Pbcn,
a
=
˚
18.5977(17), b = 10.4861(10), c = 19.4574(17) A, V = 3794.5(6)
3
A , Z = 4, qcalcd = 1.694 g cm−3, crystal dimensions: 0.40 ×
˚
0.30 × 0.30 mm3; Bruker Smart diffractometer; Mo Ka radiation,
296(2) K; 32 885 reflections, 4890 independent; (l = 1.145 mm−1);
refinement (on F2) with SHELXTL (version 6.1), 280 parameters,
0 restraints, R1 = 0.0612 (I > 2r) and wR2 (all data) =
0.1097, GOF = 1.293, max/min residual electron density:
All the isolated compounds show a single resonance in the 31
P
NMR spectra, in the 15–20 ppm chemical shift range, normal
for a metalated phosphine acting as a bridging ligand, and
consistent with a dinuclear structure. The phosphorus resonances
for compounds 7 and 8 are multiples due to long range P–F
coupling (see ESI, Fig. S11†).
−3
˚
0.832/−0.673 e A .
Electrochemical measurements
Solid state structures
All experiments were performed at 25 ◦C under an atmosphere of
dry argon in 0.10 M tetrabutylammonium hexafluorophosphate in
CH2Cl2. LSVs were obtained with a CH 1420 instrument. Glassy
carbon (geometrical area 0.071 cm2) was used as the working
electrode, a platinum wire was used as the counter-electrode,
and an aqueous AgCl (3 M NaCl)/Ag reference electrode (SCE)
separated from the bulk solution by a salt bridge containing the
solvent and supporting electrolyte only completed the standard
three-electrode cell. In these conditions, the potential of the fc+/fc
The dinuclear structure was confirmed for compounds 3–8 by
X-ray methods. The refinement of compound 3 was not fully
satisfactory, but confirmed its dinuclear structure. The structures
are of tetragonal paddle wheel type, similar to those reported for
other Pd2L4 compounds24–31 and also to the related dirhodium(II)
compound of the same stoichiometry.36
For each molecule, the core is composed of two Pd2+ ions, two
bridging metalated (C6X4)PPh2 ligands in a cisoid disposition, and
Scheme 2
5538 | Dalton Trans., 2006, 5536–5541
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two bridging carboxylate ligands as shown in Fig. 1 and 2 for two
representative examples. ORTEP diagrams37 for compounds 5, 6
and 7 are given as ESI (Fig. S1–3).† Thus, each palladium atom
is bonded to one carbon atom and one phosphorus atom, each
from a different (C6X4)PPh2 ligand, and also to two oxygen atoms
from two different carboxylate anions. These bicyclometalated
compounds with head-to-tail arrangements of phosphines are
inherently chiral. Each crystal is racemic containing the mixture
of S and R molecules, Scheme 3. The P–Pd–Pd–P torsion
angles values are between 91.2 and 98.9◦ for the characterized
compounds.
Scheme 3
In addition to the chirality (R or S) that arises from the con-
figurational arrangement of the molecule, there is a second point
of chirality in these molecules, induced by the conformational
arrangement of the ligands. The P–Pd–Pd–C torsional angles
designated with the nomenclature P and M, are shown in Scheme 4.
In each molecule, the torsion angles for the carboxylate ligands,
O–Pd–Pd–O, show the same sign as for the phosphine. Thus,
complete description of the stereochemistry in these molecules
requires specification of the two senses of chirality S or R and P
or M. In the palladium compounds described here, only molecules
with stereochemistry SM and RP have been observed, Scheme 4.
The same was observed for other structurally related rhodium(II)
compounds with metalated phosphines.36,38
Fig. 1 ORTEP diagram for compound 4. Ellipsoids represented at 30%
probability. Hydr◦ogen atoms omitted for clarity. Selected bond distances
˚
(A) and angles ( ) are: Pd(1)–Pd(2), 2.7229(8); Pd(1)–P(1), 2.2273(12);
Pd(1)–C(42), 1.982(4); Pd(1)–O(1), 2.158(3); Pd(2)–P(2), 2.2269(12);
Pd(2)–C(12), 1.980(4); Pd(2)–O(4), 2.163(3); P(1)–Pd(1)–Pd(2), 87.50(3);
P(2)–Pd(2)–Pd(1), 87.35(3).
Scheme 4
Important bond distances and angles are included in the ESI
(Table S1).† The Pd–Pd distances are in the range of values
˚
˚
2.675(1) A for 7 to 2.7229(8) A for 4. The observed trend in the
Pd–O distances deserves some comment. In all the characterized
compounds the Pd–O distances trans to carbon are slightly shorter
or similar to those trans to phosphorus. It is remarkable that for
the related dirhodium(II) compound with metalated phosphines of
general formula Rh2(PC)2(O2CR)238–44 the Pd–O distances trans to
Fig. 2 ORTEP diagram for compound 8. Ellipsoids represented at 30%
probability. Hydrogen atoms omitted for clarity. The additional “A” letter
in the atom labels indicates atoms at (2 − x, y, 1/2 − z) Selected bond
◦
˚
distances (A) and angles ( ) are: Pd(1)–Pd(1A), 2.7054(6); Pd(1)–P(1),
2.2294(11); Pd(1)–O(1), 2.116(3); Pd(1)–O(2), 2.148(3); Pd(1)–C(12),
1.987(4); P(1)–Pd(1)–Pd(1A), 89.89(3).
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carbon were considerably longer than those trans to phosphorus
and that was attributed to the higher trans influence of carbon
compared to phosphorus. While the carboxylate ligands are
symmetrically located, the metalated phosphine ligands have a
rather non-symmetric coordination. Thus, Pd–Pd–C angles, in the
95.7–93.2◦ range, are significantly bigger than the Pd–Pd–P angles
(in the 87.35–89.89◦ range of values). The same disposition had
been observed for the related dirhodium(II) compound of formula
Rh2(PC)2(O2CR)2.36,38–44 Compounds 5, 7 and 8 have crystallo-
graphically imposed twofold symmetry. Further investigations to
explain these structural trends are in progress.
pattern remains unchanged, denoting that the parent complex is
regenerated during the electrochemical turnovers.
On initiating the potential scan in the negative direction, no
reduction processes of the parent Pd complexes were detected in
the available potential region until a potential of ca. −1.8 V.
The observed electrochemistry can be described in terms of
two consecutive one-electron transfer processes in the start-
ing Pd2(+4) complexes yielding, successively, [Pd2(+5)]+ and
[Pd2(+6)]2+ cationic species. The relatively large separation between
the two associated A1/C1 and A2/C2 couples, can be described as
indicative of certain palladium–palladium interaction, as observed
in similar systems.30b,31
The cyclic voltammogram of 7 (see Fig. S10 of the ESI†)
showed two irreversible and poorly defined oxidation peaks at
approximately 1.2 and 1.7 V. Similar behaviour was observed for
compound 8.
Electrochemical study
The cyclic voltammogram of 5 in 0.10M Bu4NPF6–CH2Cl2 is
shown in Fig. 3. Compounds 3, 4 and 6 show similar electrochem-
ical behaviour (see the ESI, Fig. S4–6†). In the initial anodic scan,
two couples A1/C1 and A2/C2 appear at potential values between
+0.80 and +1.60 V. The first couple (A1/C1) can be described in
terms of a reversible one-electron transfer as judged by the value of
the anodic-to-cathodic peak separation, close to 0.059 V, recorded
at low scan rates when the potential is switched at 0.15–0.20 V
past the peak A1. The current function, Ip (= ip/Acv1/2) is close
to 450 10 mA M−1 V−1/2 s1/2 cm−2, regardless of the potential
scan rate used (0.05 < v < 0.5 V s−1), thus denoting that the elec-
trochemical process is diffusion-controlled. The formal electrode
potentials for the A1/C1 couple, calculated as the half sum of
the anodic and cathodic peak potentials for the complexes 3–6,
are +0.975, +1.250, 0.840 and 1.035 V vs. AgCl/Ag, respectively,
decreasing with the electron-donating ability of the carboxylate
group.
All these electrochemical data indicated that the presence of
electron-donating groups on the phosphine and the carboxylate
ligands favours the reversibility of the oxidation process. Thus, the
best electrochemical response was obtained for 3 and 5.
In the presence of chloride anions the voltammetric response
changes drastically as shown in Fig. 4 for compound 5. The
addition of a stoichiometric amount of Et4NCl to a solution of 5
produces the appearance of an oxidation peak (A3) at a potential
similar to that of peak A1 recorded for the parent complex. This
peak approaches an irreversible two-electron oxidation process
followed, in the subsequent cathodic scan, by a reduction peak
at 0.15 V (C4) whose anodic counterpart is entirely absent.
Consequently with these observations, the A2/C2 couple vanishes.
Compounds 3, 4 and 6 present the same behaviour with the
reduction peak shifted, as expected, to more positive values (0.28,
0.48 and 0.35 V respectively) (see Fig. S7–9 of the ESI†). Bond
et al. have reported a similar electrochemical behaviour for the
palladium(II) compounds in the presence of chlorides.30b The
observed electrochemistry can be described as a direct oxidation
of the starting Pd2(+4) complex to the [Pd2(+6)]2+ cationic species
followed by coordination of the chlorides to generate neutral
[Pd2(+6)Cl2] species.
Fig. 3 CV at glassy carbon electrode for a 1.0 mM solution of 5 in 0.10 M
Bu4NPF6–CH2Cl2.
The formal electrode potentials for the A2/C2 couple are
+1.375, +1.490, +1.310 and 1.350 V vs. AgCl/Ag, respectively.
In compounds 4 and 6, the second couple exhibits broader and
not so well defined peaks as for 3 and 5. The peak height
decreases on increasing the potential scan rate and at the same
time, are anodically shifted. These features suggest that there is
a chemical reaction preceding the electrochemical oxidation in
A2. Upon repetitive voltammetric scanning the electrochemical
Fig. 4 CV at glassy carbon electrode for 1.0 mM solution of 5 in 0.10 M
Bu4NPF6–CH2Cl2. (a) The first oxidation peak; (b) after the addition of a
stoichiometric amount of Et4N+Cl−.
5540 | Dalton Trans., 2006, 5536–5541
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17 M. Takani, H. Masuda and O. Yamauchi, Inorg. Chim. Acta, 1995, 235,
367.
Conclusions
18 M. Lenarda, G. Nardin, G. Pellizer, E. Braye and M. J. Graziani,
New palladium(II) compounds with a mixed set of metalated tri-
arylphosphines and carboxylate ligands have been characterized.
They all have a paddle wheel structure and present relatively short
palladium–palladium distances. The oxidation properties of these
compounds are very much dependent on the electron donating
properties of the metalated phosphine and the carboxylate ligands.
Thus, compounds 3–6 exhibit two reversible oxidation processes by
cyclic voltammetry, while the remaining compounds with electron-
withdrawing substituents in the metalated phosphine and/or in
the carboxylate groups lose all reversibility and only show poorly
defined oxidation processes. The ability of these compounds to
form Pd2(+6) species by chemical oxidation will be explored. The
synthetic method can be easily applied to the preparation of other
palladium compounds with selected bridging ligands. This will be
done as a second part of this study.45
J. Chem. Soc., Chem. Commun., 1985, 1536.
19 A. J. Canty, N. J. Minchin, L. M. Engelhardt and A. H. White, Inorg.
Chim. Acta, 1985, 102, L29.
20 M. Cera, E. Cerrada, M. Laguna, J. A. Mata and H. Teruel,
Organometallics, 2002, 21, 121.
21 W. L. Wilson, J. H. Nelson and N. W. Alcock, Organometallics, 1990,
9, 1699.
22 S. Jacobson, A. J. Carty, M. Mathew and G. J. Palenik, J. Am. Chem.
Soc., 1974, 96, 4331.
23 S. Kannan, A. J. James and P. R. Sharp, J. Am. Chem. Soc., 1998, 120,
215.
24 J. F. Berry, F. A. Cotton, S. A. Ibragimov, C. A. Murillo and X. P.
Wang, Inorg. Chem., 2005, 44, 6129.
25 K. Umakoshi, A. Ichimura, I. Kinoshita and S. Ooi, Inorg. Chem.,
1990, 29, 4005.
26 M. Abul-Haj, M. Quiro´s and J. M. Salas, Polyhedron, 2004, 23,
2373.
27 V. K. Jain and L. Jain, Coord. Chem. Rev., 2005, 249, 3075.
28 I. O. Koshevoy, P. Lahuerta, M. Sanau´ and M. A. Ubeda, unpublished
results.
29 M. O. Talismanova, A. A. Sidorov, G. G. Aleksandrov, Y. F. Oprunenko
and I. L. Eremenko Moiseev, II, Russ. Chem. Bull., 2004, 53,
1507.
Acknowledgements
We thank Dr F. Estevan for experimental support. We gratefully
acknowledge financial support from the MEC of Spain (project
MAT2002-0442-1-C02-02) and also for a grant to I. O. K.
30 (a) C.-L. Yao, L.-P. He, J. D. Korp and J. L. Bear, Inorg. Chem., 1988,
27, 4389; (b) A. M. Bond, A. J. Canty, J. B. Cooper, V. Tedesco, P. R.
Traill and D. M. Way, Inorg. Chim. Acta, 1996, 251, 185.
31 (a) F. A. Cotton, M. Matusz and R. Poli, Synth. Inorg. Chem., 1987,
26, 1472; (b) F. A. Cotton, M. Matusz, R. Poli and X. J. Feng, J. Am.
Chem. Soc., 1988, 110, 1144.
References
32 A. M. Aarif, F. Estevan, A. Garcia-Bernabe´, P. Lahuerta, M. Sanau´
1 V. Farina, Adv. Synth. Catal., 2004, 346, 1553.
2 B. C. G. Soderberg, Coord. Chem. Rev., 2004, 248, 1085.
3 S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400.
4 J. A. Mueller, C. P. Goller and M. S. Sigman, J. Am. Chem. Soc., 2004,
126, 9724.
5 K. C. Nicolaou, P. G. Bulger and D. Sarlah, Angew. Chem., Int. Ed.,
2005, 44, 4442.
6 A. D. Ryabov, Chem. Rev., 1990, 90, 403.
7 C. Navarro-Ranninger, F. Zamora, I. Lopez-Solera, A. Monge and J. R.
Masaguer, J. Organomet. Chem., 1996, 506, 149.
8 C. Navarro-Ranninger, F. Zamora, L. A. Martinez-Cruz, R. Isea and
J. R. Masaguer, J. Organomet. Chem., 1996, 518, 29.
9 J. L. Garcia-Ruano, A. M. Gonzalez, I. Lopez-Solera, J. R. Masaguer,
C. Navarro-Ranninger, P. R. Raithby and J. H. Rodriguez, Angew.
Chem., Int. Ed. Engl., 1995, 34, 1351.
´
and M. A. Ubeda, Inorg. Chem., 1997, 36, 6472.
´
33 F. Estevan, T. Garcia-Bernabe´, P. Lahuerta, M. Sanau´, M. A. Ubeda
and M. C. Ramirez de Arellano, Inorg. Chem., 2000, 39, 5964.
34 M. F. M. Rettig, Inorg. Synth., 1977, 17, 134.
35 P. G. Eller and D. W. Meek, J. Organomet. Chem., 1970, 22, 631.
36 A. R. Chakravarty, F. A. Cotton, D. A. Tocher and J. H. Tocher,
Organometallics, 1985, 4, 8.
37 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
´
38 F. Estevan, P. Lahuerta, E. Peris, M. A. Ubeda, S. Garc´ıa-Granda,
F. Go´mez-Beltra´n, G. Gonza´lez, E. Pe´rez-Carren˜o and M. Martinez,
Inorg. Chim. Acta, 1994, 218, 189.
39 M. Barberis, P. Lahuerta, J. Pe´rez-Prieto and M. Sanau´, Chem.
Commun., 2001, 439.
40 F. Estevan, P. Lahuerta, J. Latorre, E. Peris, S. Garc´ıa-Granda, F.
Go´mez-Beltra´n, A. Aguirre and M. A. Salvado´, J. Chem. Soc., Dalton
Trans., 1993, 1681.
10 N. Masciocchi, F. Ragaini and A. Sironi, Organometallics, 2002, 21,
3489.
´
41 F. Estevan, P. Lahuerta, E. Peris, M. A. Ubeda, S. Garc´ıa-Granda,
11 G. L. Tian, P. D. Boyle and B. M. Novak, Organometallics, 2002, 21,
F. Go´mez-Beltra´n, E. Pe´rez-Carren˜o, G. Gonza´lez and M. Mart´ınez,
Inorg. Chim. Acta, 1994, 218, 189.
42 F. P. Pruchnik, R. Starosta, P. Smolenski, E. Shestakova and P.
Lahuerta, Organometallics, 1998, 17, 3684.
43 F. P. Pruchnik, R. Starosta, T. Lis and P. Lahuerta, J. Organomet.
Chem., 1998, 568, 177.
1462.
12 J. Vicente, A. Arcas, J. M. Ferna´ndez-Herna´ndez and D. Bautista,
Organometallics, 2001, 20, 2767.
13 J. Ruiz, V. Rodriguez, N. Cutillas, M. Pardo, J. Pe´rez, G. Le´pez, P.
Chaloner and P. B. Hitchcock, Organometallics, 2001, 20, 1973.
14 K. Nakatsu, K. Kafuku, H. Yamaoka, K. Isobe, Y. Nakamura and S.
Kawaguchi, Inorg. Chim. Acta, 1981, 54, L69.
15 D. A. Culkin and J. F. Hartwig, J. Am. Chem. Soc., 2002, 124, 9330.
16 J. Ruiz, V. Rodriguez, G. Lopez, J. Casabo, E. Molins and C. Miravitlles,
Organometallics, 1999, 18, 1177.
44 F. Estevan, P. Krueger, P. Lahuerta, E. Moreno, J. Pe´rez-Prieto, M.
Sanau´ and H. Werner, Eur. J. Inorg. Chem., 2001, 105.
45 F. A. Cotton, I. O. Koshevoy, P. Lahuerta, C. A. Murillo, M. Sanau´,
´
M. A. Ubeda and Q. Zhao, J. Am. Chem. Soc., in press.
This journal is
The Royal Society of Chemistry 2006
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