Electrochemical reduction of palladium-containing heteronuclear complexes

Article abstract of DOI:10.1023/B:RUGC.0000015972.76679.7a

Reduction of a series of palladium-containing complexes with MnPd, Fe ₂Pd, and Fe₃Pd skeletons was studied by classical polarography, cyclic voltammetry, and controlled-potential electrolysis in acetonitrile. The electrochemical behavior of these complexes was compared to that of relared platinum complexes.

Full text of DOI:10.1023/B:RUGC.0000015972.76679.7a

Russian Journal of General Chemistry, Vol. 73, No. 9, 2003, pp. 1335 1340. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 9, 2003,
pp. 1413 1418.
Original Russian Text Copyright
2003 by Burmakina, Pogrebnyakov, Pavlenko, Antonova, Rubailo.
Electrochemical Reduction
of Palladium-containing Heteronuclear Complexes
G. V. Burmakina, D. A. Pogrebnyakov, N. I. Pavlenko,
A. B. Antonova, and A. I. Rubailo
Institute of Chemistry and Chemical Technology, Siberian Division, Russian Academy of Sciences, Krasnoyarsk, Russia
Received November 1, 2001
Abstract Reduction of a series of palladium-containing complexes with MnPd, Fe2Pd, and Fe3Pd skeletons
was studied by classical polarography, cyclic voltammetry, and controlled-potential electrolysis in acetonitrile.
The electrochemical behavior of these complexes was compared to that of relared platinum complexes.
Palladium complexes are widely used as catalysts
of redox reactions, for example, oxidation of alkenes
into carbonyl compounds and their acetals and ketals,
oxidative acetoxylation of alkenes and alkylarenes,
reduction of multiple bonds in alkynes and alkenes,
etc. [1 3]. The mechanisms of such reactions are not
completely clear, which makes difficult selecting
optimal conditions and catalysts. To elucidate the
mechanism and to develop certain catalytic processes,
it is useful to know features of redox transformations
of palladium complexes, which are readily established
by electrochemical methods.
Ia, Ib IIIa, and IIIb contain a single one-electron
reduction wave (Fig. 1, curve 1 and table).
H
Ph
C
C
O
L
C
M Mn
L
C
O
Ia, Ib IIIa, IIIb
M = Pd (Ia, IIa, IIIa), Pt (Ib, IIb, IIIb); LL = 2 PPh3
(Ia, Ib), dppe (IIa, IIb), dppp (IIIa, IIIb).
In the present work we performed electrochemical
reduction of three types of palladium intermetallic
As seen from the table, replacement of two mono-
dentate ligands PPh3 at Pd or P in complexes Ia and
Ib by chelate diphosphine ligands dppe or dppp shifts
the reduction potentials of complexes IIa, IIb, IIIa,
and IIIb to the cathodic region in parallel with the
change of the electron-donor power of the new ligands.
Complexes IIa, IIb, IIIa, and IIIb are reduced re-
versibly in contrast to compounds Ia and Ib (the
ipa/ipc ratio in cyclic voltammetry on a hanging
mercury drop is close to unit).
complexes: binuclear MnPd compounds, such as
2
Cp(CO)2MnPd( -C=CHPh)L2 [L2 = 2PPh3 (Ia),
Ph2P(CH2)2PPh2(dppe) (IIa),
-
2-Ph2P(CH2)3PPh2
(dppp) (IIIa)], trinuclear Fe2Pd complex (CO)8Fe2
Pd(dppe) (IVa), and tetranuclear clusters (CO)9Fe3
Pd( 4-C=CHPh)( 2-P P) [P P = dppe (Va), dppp
(VIa)].
The electrochemical behavior of compounds Ia
VIa was compared with the behavior of their platinum-
containing analogs Ib VIb which we studied earlier
[4 6] under the same conditions.
A feature of the electrochemical behavior of bi-
nuclear platinum and palladium complexes with the
dppe and dppp ligands is worth noting. The reduction
half-wave potentials of both palladium (IIa, IIIa) and
platinum (IIb, IIIb) complexes do not change as the
size of the chelate ring increases by one CH2 group
(see table). Moreover, the half-wave potentials E1/2 of
all the four platinum and palladium dppe and dppp
complexes IIa, IIb, IIIa, and IIIb are the same
( 2.54 eV).
The electrochemical characteristics of respective
palladium and platinum complexes Ia VIa and Ib
VIb are given in the table. The reduction waves
whose parameters are given in the table are diffusion-
controlled, as their limiting currents ilim are propor-
4
tional to the concentration of complexes c (1 10
3
1/2
1 10 M), h
[h is the height of the mercury
1/2
column (25 65 cm)], and to V
[V is the potential
At the same time, replacement of platinum in
complex Ib with terminal PPh3 ligands by palladium
shifts the reduction potential of the resulting complex
scan rate (0.01 0.5 V/s)].
The classical polarograms of binuclear complexes
1070-3632/03/7309-1335$25.00 2003 MAIK Nauka/Interperiodica

1336
BURMAKINA et al.
Electrochemical characteristics palladium and platinum containing heteronuclear complexes (dropping mercury electrode,
3
MeCN, 0.1 M Et4NBF4, c 1 10 M, Ag/0.1 M AgNO3 in MeCN)
First wave
E1/2, V
Binuclear complexes
Second wave
Third wave
Comp.
no.
Compound
n
E1/2, V
n
E1/2, V
n
Ia
Ib
IIa
IIb
IIIa
IIIb
Cp(CO)2MnPd( -C=CHPh)(PPh3)2
Cp(CO)2MnPt( -C=CHPh)(PPh3)2
Cp(CO)2MnPd( -C=CHPh)(dppe)
Cp(CO)2MnPt( -C=CHPh)(dppe)
Cp(CO)2MnPd( -C=CHPh)(dppp)
Cp(CO)2MnPt( -C=CHPh)(dppp)
2.24
1
1
1
1
1
1
2.37
2.54a
2.54a
2.54a
2.54a
Trinuclear complexes
IVa
IVb
(CO)8Fe2Pd(dppe)
(CO)8Fe2Pt(dppe)
1.40a
1.64a
2
2
2.06
1.96
1
1
2.30
2.72b
2.66
2.81
1
2
1
1
Tetranuclear complexes
Va
(CO)9Fe3Pd( 4-C=CHPh)(dppe)
(CO)9Fe3Pt( 4-C=CHPh)(dppe)
(CO)9Fe3Pd( 4-C=CHPh)(dppp)
(CO)9Fe3Pt( 4-C=CHPh)(dppp)
1.14a,b
1.06
2
1
1
2
2.44
2
1.20
Vb
1.43a
2.72b
2.66
2.81
2.44
2
1
1
2
VIa
VIb
1.14a,b
1.06
2
1
1
2
1
1
1.20
1.43a,b
1.33
2.72b
2.63
2.80
2
1.55
a
b
Reversible stage. At lower potential scan rates, the two-electron wave splits into two one-electron waves whose potentials are
printed italic.
Ia to the anodic region, which agrees with the change
in the electron-acceptor power of the metal.
reduction of complexes IIa, IIb, IIIa, and IIIb points
to formation of electron-transfer products, i.e. radical
anions stable in the time scale of cyclic voltammetry.
In our opinion, the anomalous behavior of palla-
dium and platinum binuclear complexes IIa, IIb, IIIa,
and IIIb with chelate diphosphine ligands in terms of
the electronic effects on their electrochemical charac-
teristics can be explained as follows: Introduction of
dppe or dppp ligands results in formation of a stable
five- or six-membered ring M P(CH2)n P involving
Pd or Pt. Therewith, the nature of one of the CO
groups at the Mn atom changes, namely, it becomes
semibridging [7]. The formation of such rings gives
rise to electron density delocalization in the molecules,
which reveals itself in the equality of the E1/2 poten-
tials of complexes IIa, IIb, IIIa, and IIIb and in the
reversibility of their one-electron reduction. It was
established by cyclic voltammetry that the reversible
The polarograms of binuclear complexes Ia, Ib
IIIa, and IIIb in MeCN solutions subjected to electro-
lysis on a mercury bottom at controlled potential in
the region of the limiting current of one-electron re-
duction wave show one reduction wave at E1/2 ca.
2.80 V (reduction begins at 2.60 V, the limiting
current corresponds to addition of more than two elec-
trons). This wave overlaps with the reduction wave of
the background electrolyte (E1/2 3.20 V) (Fig. 1,
curve 2). The IR spectra of these solutions, as well as
TLC data point to appearance of cymantrene
CpMn(CO)3 and phenylacetylene HC CPh in these
solutions, which are reduced at E1/2 2.70, 2.90 and
2.80, 2.93 V, respectively. Therefore, the reduction
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ELECTROCHEMICAL REDUCTION
1337
2
1
2
1
id
id
1
2
1 A
3
3
1 A
0
1.0
2.0
3.0 E, V
3
1
2
0
1.0
2.0
3.0 E, V
Fig. 1. Current voltage curves for the Cp(CO) MnPd
2
( C=CHPh)(dppe) complex (1) before electrolysis and
Fig. 2. Polarograms of the (CO) Fe Pd(dppe) cluster
8 2
(2) after electrolysis at E 2.60 V; (3) background curve
(1) before electrolysis and (2) after electrolysis at E
e
e
3
(MeCN, 0.1 M Et NBF , c 1 10
M, Ag/0.1 M
1.50 V; (3) background curve (MeCN, 0.1 M Et NBF ,
4 4
4
4
3
AgNO in MeCN).
c 1 10 M, Ag/0.1 M AgNO in MeCN).
3
3
waves at E1/2
2.80 V in the polarograms of solu-
Trinuclear cluster IVa is reduced in three stages at
E1/2 1.40, 2.06, and 2.30 V (Fig. 2, curve 1; see
table) with addition of two electrons in the first stage
and of one electron in the two subsequent stages. The
first two-electron stage is reversible, as shown by
cyclic voltammetry.
tions of complexes Ia, Ib IIIa, and IIIb after elec-
trolysis can be considered as overlapping reduction
waves of cymantrene and phenylacetylene formed by
electrolysis.
Thus, combined results of classical polarography
and electrolysis of binuclear platinum and palladium
complexes Ia, Ib IIIa, and IIIb allow the following
assumptions as to the mechanism of the corresponding
reactions. The one-electron reduction of the com-
plexes gives radical anions.
PPh2
Ph2P
M
O
O
C
C
Fe
Fe
(CO)3 (CO)3
IVa, IVb
Cp(CO)2MnM( -C=CHPh)L2 + 1e
M = Pd (IVa), Pt (IVb).
Ia, Ib IIIa, IIIb
[Cp(CO)2MnM( -C=CHPh)L2]
Unlike complex IVa, its platinum analog IVb takes
up two electrons in the third stage (see table) [5]. The
presence of a two-electron reduction wave indicates
that the difference in the potentials of the first and
second electron transfers is small. At low potential
scan rates, the third two-electron reduction wave of
complex IVb splits into two one-electron waves, and
thus four waves appear in the polarogram (see table).
The radical anions of the complexes with chelate
dppe and dppp ligands are more stable than those of
the complexes with terminal triphenylphosphine
ligands, which is evidenced by the reversible reduc-
tion of the former. Our results agree with the conclu-
sions in [8 10]: Binuclear complexes containing a
metal metal bond and bridging ligands are reduced
reversibly to give products whose stability varies over
a wide range.
The E1/2 value for the first reduction wave of
complex IVa is shifted to the anodic region, and that
for the second wave, to the cathodic region compared
to respective values for complex IVb. This fact may
imply that further reduction of the dianions of com-
plexes IVa and IVb, formed in the first two-electron
reduction stage, occurs differently.
The electrolysis of all the binuclear complexes
results in rupture of the Mn M bonds (M = Pd, Pt)
and formation of cymantrene, phenylacetylene, and
palladium- or platinum-containing fragments.
Cp(CO)2MnM( -C=CHPh)L2 + e
[Cp(CO)2MnM( -C=CHPh)L2]
The polarograms of tetranuclear clusters Va and
VIa are identical and contain two two-electron reduc-
tion waves at E1/2 1.14 and 2.44 V (Fig. 3, curve 1;
CpMn(CO)3 + HC CPh + M-containing fragments.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 9 2003

1338
BURMAKINA et al.
As noted above, not only the reduction characte-
H
ristics (reversibility and equal number of electrons
added, n = 1) of binuclear complexes Ia, Ib IIIa, and
IIIb, but also the E1/2 values are independent of the
nature of the diphosphine ligand and metal M. This
can be caused by the presence of a chelate ring formed
by the dppe or dppp ligand and M and of the semi-
bridging CO group, which results in electron density
delocalization in the molecule. The tetranuclear pla-
tinum clusters comprise, along with the dppe and
dppp chelates, two semibridging CO groups [11], in
contrast to the palladium compounds where the semi-
bridging M CO bond is weak [12]. This is a probable
reason for the different effect of the metals on the
reduction potentials of complexes Va, VIa and Vb,
VIb.
C
Ph
P
P
(
C
M
Fe(CO)3
Fe
Fe
(CO)3 (CO)3
Va, Vb, VIa, VIb
M = Pd (Va, VIb), Pt (Vb, VIb). P P = dppe (Va, Vb),
dppp (VIa, VIb).
see table), the first of them being reversible, as shown
by cyclic voltammetry. Their platinum analogs Vb
and VIb, too, are reduced in two two-electron stages
at equal potentials of respective waves (E1/2 1.43 and
2.72 V). In certain cases, at low potential scan rates,
the two-electron waves split into two one-electron
waves (their potentials are given in the table under
the values for corresponding two-electron waves and
printed italic).
To elucidate the mechanisms of the electrochemical
reduction of the tri- and tetranuclear clusters, we
performed electrolysis of solutions of complexes IVa,
IVb VIa, and VIb in acetonitrile on a mercury
bottom at controlled potential of the limiting current
of the first two-electron reduction wave.
The E1/2 values for the reduction of palladium-
containing clusters Va and VIa are shifted to the
anodic region compared to those for platinum-contain-
ing clusters Vb and Vb. Therewith, the differences
between the E1/2 values for the first and second two-
electron reduction waves of both clusters Va, VIa,
and Vb, VIb are equal and make 1.3 V.
The polarogram of a solution of trinuclear cluster
IVa subjected to electrolysis (Fig. 2, curve 2) contains
a reduction wave at E1/2 2.44. This wave is present
in the classical polarograms of the tetranuclear palla-
dium clusters (see table), but it was not detected in the
polarograms of similar platinum compounds. At the
same time, the reduction wave at E1/2 2.72 V, which
is absent from the polarograms of solutions of the
palladium clusters, is observed for the tri- and tetra-
nuclear platinum clusters (see table). This suggests
that the reduction wave at E1/2 2.44 V belongs to a
palladium-containing fragment, and that at E1/2
2.72 V, to a platinum-containing fragment, which are
formed after two-electron reduction of clusters IVa
and IVb, respectively.
Thus, the electrochemical characteristics of clusters
(CO)9Fe3M( -C=CHPh)( 2-P P) Va, Vb, VIa, and
4
VIb depend on the nature of metal M, but do not
depend on the nature of the chelate ligand at this atom.
2
1
Except for the reduction waves of palladium- and
id
platinum-containing fragments (E1/2
2.44 and
2.72 V, respectively), new reduction waves were
detected at E1/2 0.81 and 1.12 V in the polarograms
of the solutions of both palladium and platinum-con-
taining tetranuclear clusters Va, VIa and Vb, VIb
after electrolysis. These waves have the same E1/2
values as the reduction waves of the trinuclear cluster
(CO)9Fe3( 3-C=CHPh) (VII) [13]. This fact suggests
formation of cluster VII after two-electron reduction
of compounds Va, VIa and Vb, VIb. Cluster VII is
reduced at higher positive potentials than clusters Va,
Vb, VIa, and VIb (see table). Therefore, no reduction
waves of of cluster VII were found in the polarograms
of the solutions of clusters Va, Vb, VIa, and VIb
after electrolysis.
3
1 A
0
2
1.0
2.0
3.0 E, V
1,3
Fig. 3. Polarograms of the (CO) Fe Pd( -C=CHPh)
(dppe) cluster (1) before and (2) after electrolysis at E
1.30 V; (3) background curve (MeCN, 0.1 M Et NBF ,
c 1 10 M, Ag/0.1 M AgNO in MeCN).
9
3
4
e
4
4
3
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 9 2003

ELECTROCHEMICAL REDUCTION
1339
Thus, tetranuclear clusters Va and VIa are reduced
with reversible addition of two electrons to give cor-
responding dianions whose decomposition is likely to
provide trinuclear cluster VII and palladium-contain-
ing fragments. Further these fragments are reduced at
E1/2 2.44 V to Pd(0) which precipitates on the elec-
trode, as evidenced by the observation of a gray pal-
ladium film on the mercury surface.
mograms were recorded on a PA-2 polarograph
operated in the three-electrode mode. The potential
scan rate was varied within 0.01 0.50 V/s. The work-
ing electrodes were a dropping mercury electrode with
forced dropping (m 3.6 mg/s, 0.23 s) or a stationary
platinum electrode (diameter 1 mm) embedded in a
Teflon housing (10 mm). For reproducible operation
the platinum working electrode was polished before
each measurement with a wet aluminum oxide powder
(chromatographic grade) applied on a felt support.
On the basis of the aforesaid we can propose the
following scheme of the electrochemical reduction of
tetranuclear clusters Va, Vb, VIa, and VIb.
The reference electrode was Ag/0.1 M AgNO3 in
MeCN, connected with the cell by an electrolytic
bridge filled with the background electrolyte. The
auxiliary electrode was a platinum spiral placed in a
glass tube with a porous filter.
(CO)9Fe3M( 4-C=CHPh)( 2-P P) + 2e
Va, Vb, VIa, VIb
[(CO)9Fe3M( 4-C=CHPh)( 2-P P)]2
The number of electrons participating in each
reduction stage was determined by polarographic
coulometry [17].
(CO)9Fe3( 3-C=CHPh) + M-containing fragments
VII
e
2e
Controlled-potential electrolysis was carried out
on a PA-3 polarograph operated in the three-electrode
mode. The working electrode was bottom mercury
(4 cm2) located on the bottom of the electrochemical
cell. Other experimental conditions were the same as
above-described. Electrolysis was carried out with
vigorous stirring and bubbling argon.
[(CO)9Fe3( 3-C=CHPh)]
e
M(0) + fragments
[(CO)9Fe3( 3-C=CHPh)]2
The electrochemical study of bi-, tri, and tetranuc-
lear palladium-containing compounds and comparison
of their behavior with the behavior of their platinum-
containing analogs showed that introduction of chelate
dppe or dppp ligands instead of two mononuclear
PPh3 ligands at the Pd or Pt atom in binuclear com-
plexes makes reduction of the resulting complexes
reversible, i.e. increases stability of species formed by
reduction. Increase in the number of metal atoms
facilitates reduction and increases stability of tri- and
tetranuclear clusters, compared to binuclear clusters,
in redox reactions, which leads to reversibility of their
two-electron reduction. The difference in the electro-
chemical characteristics of platinum and palladium
tetranuclear clusters with the dppe and dppp ligands is
apparently connected with the presence of two semi-
bridging CO groups in the platinum-containing
clusters and weak M CO bonds in the palladium-
containing analogs.
ACKNOWLEDGMENTS
This work was financially supported by the Rus-
sian Foundation for Basic Research (project no. 99-
03-33027) and INTAS (grant no. 00-291).
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