Comparison of electrochemical behavior of exohedral palladium complexes with [60]- and [70]-fullerenes and metallocene ligands

Article abstract of DOI:10.1023/B:RUCB.0000037844.93864.3c

Electronic structures of exohedral palladium complexes of [60]- and [70]-fullerenes with diphenylphosphinoferrocenyl, diphenylphosphinoruthenocenyl, and diphenylphosphinocymantrenyl ligands were studied by cyclic voltammetry and semiempirical quantum-chem

Full text of DOI:10.1023/B:RUCB.0000037844.93864.3c

Russian Chemical Bulletin, International Edition, Vol. 53, No. 4, pp. 795—799, April, 2004
795
Comparison of electrochemical behavior of exohedral palladium complexes
with [60]ꢀ and [70]ꢀfullerenes and metallocene ligands
T. V. Magdesieva,aꢀ V. V. Bashilov,b D. N. Kravchuk,a F. M. Dolgushin,b K. P. Butin,a and V. I. Sokolovb
aDepartment of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (095) 939 0290. Eꢀmail: tvm@org.chem.msu.ru
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085
Electronic structures of exohedral palladium complexes of [60]ꢀ and [70]ꢀfullerenes with
diphenylphosphinoferrocenyl, diphenylphosphinoruthenocenyl, and diphenylphosphinoꢀ
cymantrenyl ligands were studied by cyclic voltammetry and semiempirical quantumꢀchemical
calculations. Probable sites of localization of electronic changes in the molecules of these
complexes under electrochemical oxidation and reduction were determined.
Key words: fullerenes, exohedral palladium complexes, cymantrene, ruthenocene, elecꢀ
tronic structure, voltammetry, semiempirical calculations.
We have recently1,2 synthesized a series of new exoꢀ
hedral Pd0 complexes with [60]ꢀ and [70]ꢀfullerenes
containing 1,1´ꢀbisdiphenylphosphinoferrocene (dppf),
1,1´ꢀbisdiphenylphosphinoruthenocene (dppr), or two
diphenylphosphinocymantrene (dppcym) molecules as
stabilizing ligands. These complexes contain a strongly
electronꢀwithdrawing fullerene cage and a metallocene
group, which can be either electronꢀreleasing (ruthenoꢀ
cene) or electronꢀwithdrawing (cymantrene) and is linked
with the cage through the bisdiphenylphosphinepalladium
bridge. Intramolecular charge transfer can occur between
these groups and the fullerene core and should affect the
electrochemical reduction potentials (E Red) of the fulleꢀ
rene moiety (C60, C70), which is active in the cathodic
potential region, and oxidation potentials (E Ox) of the
metallocene group, which is active in the anodic region.
The electrochemical pattern is impeded by the fact that
the bisdiphenylphosphinepalladium fragment linking these
terminal groups is also redoxꢀactive.
Experimental
The C60Pd(dppf), C60Pd(dppcym), C60Pd(dppr),
C70Pd(dppcym), and C70Pd(dppf) complexes were synꢀ
thesized using a Schlenk technique by a previously deꢀ
scribed method.1,2 The latter includes the reaction of equivaꢀ
lent amounts of the respective fullerene, Pd2(dba)3 complex
(dba is dibenzylideneacetone), and phosphine ligand under
argon.
Measurements of E Ox and E Red were carried out using a
digital IPC Win potentiostatꢀgalvanostat. Voltammograms were
recorded with 0.15 М Bun4NBF4 as a supporting electrolyte in
oꢀdichlorobenzene at 20 °C in a 10ꢀmL electrochemical cell.
Oxygen was removed with dry argon passed through the cell.
The CV curves were recorded on a stationary graphite electrode
with a sweep rate of 100 and 200 mV s–1. Potentials of peaks,
which were often poorly pronounced in CV curves, were deterꢀ
mined by a special program installed in the IPC Win program
package. Measured values were recalculated taking into account
ohmic losses.
Pyrolyzed polyacrylonitrile (PAN) with a specific surface of
12 m2 g–1 was used as the material for a working electrode. An
auxiliary electrode was platinum, and a saturated Ag/AgCl elecꢀ
trode was a reference electrode.
Calculations were performed by the ZINDO/1 method inꢀ
cluded in the HyperChem program package.
оꢀDichlorobenzene (reagent grade) was purified with calꢀ
cium hydride for 4 h and distilled in vacuo collecting a fraction
with b.p. 48 °C (10 Torr). Prior to use, the supporting electroꢀ
lyte Bun4NBF4 was twice recrystallized from anhydrous toluene
and dried.
In this work, we studied and compared the
С60Pd(dppf),
C60Pd(dppr),
C60Pd(dppcym)2,
C70Pd(dppr), and C70Pd(dppcym)2 complexes by cyclic
voltammetry (CV) and semiempirical quantumꢀchemical
calculations using available Xꢀray diffraction data to eluꢀ
cidate the electronic structures of these complexes and to
reveal redox sites (i.e., sites of localization of electronic
changes) responsible for the electrochemical reduction
and oxidation. The data obtained can be helpful for evaluꢀ
ation of these compounds to be used as electroꢀ and
photoactive materials, catalysts, etc.
Qualitative measurements of absorbance in the electronic
spectra (Specord UV—Vis) for a solution of C60Pd(dppcym)2
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 759—763, April, 2004.
1066ꢀ5285/04/5304ꢀ0795 © 2004 Plenum Publishing Corporation

796
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 4, April, 2004
Magdesieva et al.
were carried out in an open cell with periodical sampling of the
solution, which contacted with air.
I/µA
a
b
c
40
Results and Discussion
20
0
All complexes under study are scarcely soluble in the
most part of solvents. Therefore, only oꢀdichlorobenzene
(DCB) and a toluene—MeCN (4 : 1, vol/vol) mixture
turned out to be appropriate for electrochemical studies;
however, even in this case, the concentration of the comꢀ
plexes did not exceed 1•10–4 mol L–1. Moreover, the
complexes are unstable in solutions. The cymantrene
complexes, whose solutions contain a mixture of prodꢀ
ucts including free fullerenes, are especially unstable.
The dynamics of changing the electronic spectrum of
C60Pd(dppcym)2 in toluene in the presence of oxygen
traces is presented in Fig. 1. The intensity of the characꢀ
teristic absorption bands of C60Pd(dppcym)2 at 22780,
16260, and 15260 cm–1 decreases sharply, which indiꢀ
cates that the compound is unstable under these condiꢀ
tions. Fullerene is a product of C60Pd(dppcym)2 deꢀ
composition. The other products of decomposition of
C60Pd(dppcym)2 in air were not studied in this work.
Using PAN as a working electrode, one can obtain
more pronounced peaks of redox transitions than those,
e.g., when smooth Pt is used.
The voltammograms for the exohedral palladium comꢀ
plexes of С60 with the ferrocenylꢀ, ruthenocenylꢀ, and
cymantrenylꢀcontaining phosphine ligands are presented
in Fig. 2.
The CV curves of the complexes are similar. The shape
of the curves is complicated because the molecules conꢀ
tain three redoxꢀactive sites. In addition, a possible parꢀ
tial dissociation of the complexes in solutions and their
oxidation with oxygen traces often produce many peaks
–20
–2
I/µA
80
–1
0
1
E/V
60
40
20
0
–20
–2
I/µA
–1
0
1
E/V
50
0
–50
–2
–1
0
1
E/V
64 min
0 min
Fig. 2. Voltammograms of the heterobimetallic complexes of С60
:
С60Pd(dppf) (a), С60Pd(dppr) (b), and С60Pd(dpcym)2 (c)
(PAN, оꢀdichlorobenzene, 0.15 М Bu4NBF4, vs. Ag/AgCl/KCl,
200 mV s–1).
22780
0 min
15 min
20 min
30 min
57 min
64 min
in the CV curves, especially in the cathodic region, due to
which their interpretation is often difficult.
16260
0 min
15270
0 min
The reduction potentials of C60Pd(dppcym)2 and
C70Pd(dppcym)2, which were measured by us earlier3
and virtually coincided with the reduction potentials of
free C60 and C70, are incorrect, probably, because of
partial destruction of the complexes. The data given in
Table 1 are correct. In special experiments, we succeeded
to obtain (on PAN) well discernible reduction peaks for a
mixture of C60Pd(dppcym)2 and C60 in a ratio of 1 : 2
(Fig. 3). The use of vacuum in preparing solutions of
ν/cm–1
Fig. 1. Time changes in the electronic absorption spectrum of
60Pd(dppcym)2 in the presence of oxygen traces in a toluene
solution.
C

Electrochemistry of exohedral fullerene complexes
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 4, April, 2004
797
Table 1. Oxidation (EOx) and reduction (ERed) potentials of the
СnМL2 complexes (M = Pd, Pt; L2 = dppf, dppr, (dppcym)2;
n = 60, 70) and free fullerenes*
For all complexes, the E Red values are shifted to the
cathodic region compared to E Red of free fullerenes (see
Fig. 3), which can be interpreted as a consequence of
charge transfer from the metalꢀcontaining fragment to
the fullerene moiety of the molecule.
Compound
E Ox
–E Red
The first peak in the voltammograms in the anodic
region for all complexes is irreversible and corresponds to
twoꢀelectron oxidation. The E Ox values of this redox
transition lie within the interval from 0.85 to 1.05 V
(vs. Ag/AgCl/KCl), depending on the nature of the
metallocenylphosphine ligand. In the case of С60Pd(dppf)
and С60Pd(dppr), the second anodic peak is observed at
potentials close to the oxidation potentials of the respecꢀ
tive 1,1´ꢀmetallocenyl(bisdiphenylphosphinepalladium)
dichlorides (e.g., 1.22 V for С60Pd(dppf) and 1.20 V for
Pd(dppf)Cl2).
V
2
η ꢀC60Pd(dppf)
0.87, 1.22
0.82, 1.16
1.03, 1.44
0.86, 1.20
1.03, 1.35
—
0.69, 1.04, 1.52
0.61, 0.96, 1.34
0.60, 0.97, 1.38
0.63, 0.98, 1.40
0.61, 0.99, 1.41
0.41, 0.78, 1.20
0.41, 0.77, 1.19
2
η ꢀC60Pd(dppr)
2
η ꢀC60Pd(dppcym)2
2
η ꢀC70Pd(dppr)
2
η ꢀC70Pd(dppcym)2
C60
C70
—
* Experimental conditions: DCB, PAN electrode, 0.15 М
Bu4NBF4, vs. Ag/AgCl/KCl.
Based on the CV data, we can propose the following
sequence of the initial steps of electrochemical oxidation
of С60Pd(dppf) and С60Pd(dppr) (Scheme 1).
C60Pd(dppcym)2 and C70Pd(dppcym)2 also made it posꢀ
sible to detect pronounced reduction peaks of the indiꢀ
vidual complexes in the voltammograms (see Fig. 2).
I/µA
Scheme 1
40
a
30
20
10
0
*
*
*
*
–10
–20
–30
*
It is likely that the twoꢀelectron oxidation of the comꢀ
plex at E1Ox mainly concerns Pd, and the dication formed
dissociates rapidly to free fullerene and [PdIIL2]2+ diꢀ
cation, which is probably coordinated with the solvent
*
–1
0
1
E/V
Ox
I/µA
molecules. The latter peak at E2 is attributed to the
oxidation of the metallocene moiety of the complex.
To verify the above assumptions on localization of the
redoxꢀactive sites in the complexes, we performed semiꢀ
empirical calculations of frontier orbital energies by
the ZINDO/1 method4,5 included in the HyperChem
program package (HyperCube Inc., FL, USA). The
geometry of molecules was optimized by the ZINDO/1
method for singlet states using a restricted Hartree—Fock
theory. Geometry optimization was performed with the
b
50
*
*
0
*
*
fixed convergence gradient at most 10 cal Å–1 mol–1
.
*
In the HyperChem program, parameters were used by
default.
–50
*
The semiempirical calculations showed that the lowest
unoccupied molecular orbital (LUMO) for the C60PdL2
complexes (L2 = dppf, dppr, (dppcym)2) is localized on
the fullerene cage, and the metalꢀcontaining fragment
makes virtually no contribution to this orbital (Fig. 4).
For the complexes with L2 = dppf and dppr, the main
contribution to the formation of the highest occupied
–1
0
1
E/V
Fig. 3. Voltammograms of the С60Pd(dppcym)2—С60 (a) and
С60Pd(dppr)—С60 (b) mixtures in oꢀdichlorobenzene (PAN,
0.15 М Bu4NBF4, vs. Ag/AgCl/KCl, 200 mV s–1). Peaks of С60
are marked by asterisks.

798
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 4, April, 2004
Magdesieva et al.
LUMO
HOMO
Table 2. Lengths (d) of the shortest C—C bond in fullerenes C60
and C70 and the C—C metalꢀcoordinating bond in CnML2
a
Compound
d/Å
Ref.
C60Pt(PPh3)2
1.502(30)
1.447(25)
1.472(7)
1.476(9)
1.476(9)
1.39
7
8
C
60Pd(PPh3)2
C60Pd(dppcym)2
60Pd(dppr)
2, this work
2, this work
2, this work
9
C
C70Pd(dppr)
C60
C70
1.380
10
b
E
Red/V
6
–0.40
–0.45
–0.50
–0.55
–0.60
–0.65
–0.70
7
5
c
4
3
2
1
1.38 1.40 1.42 1.44 1.46 1.48
dC—C/Å
Fig. 5. Relationship between the C(1)—C(2) bond length (dC—C
)
2
and the reduction potential (ERed) of the η ꢀfullerene comꢀ
Fig. 4. LUMO and HOMO calculated for C60Pd(dppf) (a),
C60Pd(dppr) (b), and C60Pd(dppcym)2 (c).
plexes: С60Pt(PPh3)2 (1), С60Pd(dppcym)2 (2), С60Pd(dppr) (3),
С
70Pd(dppr) (4), С60Pd(PPh3)2 (5), C60 (6), and C70 (7).
molecular orbital (HOMO) is made by the orbitals of Pd
involving the ferrocene or ruthenocene fragment. In the
case of the cymantrenylpalladium complex, the HOMO
is distributed over the P—Pd—P fragment involving no
cymantrenyl groups (unlike two preceding complexes).
Thus, the electrochemical potentials and results of calcuꢀ
lations agree. Therefore, one can conclude that reduction
primarily involves the fullerene orbitals, whereas oxidaꢀ
tion involves the orbitals of palladium.
The Xꢀray diffraction data on the complexes* showed
that the Pd atom coordinated with the double bond of
fullerene and two P atoms has the planar configuration,
which agrees with the published data6 for other similar
complexes. The C—C bond in the palladocycle elongates
considerably upon coordination compared to that in free
fullerene (Table 2), and the degree of pyramidality of the
C atoms bound to Pd also changes.
stantially the reduction potentials of the complexes. The
exohedral fullerene complexes exhibit an interesting relaꢀ
tionship between the electrochemical and Xꢀray diffracꢀ
tion data. It is more difficult to reduce the exohedral
fullerene complex with a longer C—C bond (Fig. 5). In
other words, the stronger an extension of the C=C bond
of the fullerene cage upon coordination, the higher an
increase in the LUMO energy and the more difficult the
reduction of the complex. The correlation shown in Fig. 5
(r = 0.996) is described by a linear equation
E
Red = –2.37dC—C + 2.88,
i.e., the potential values are rather sensitive to a change in
the geometry (correlation coefficient 2.37 V Å–1).
Red
The correlation of E1
with the geometry of the
complexes shows the sensitivity of the electron affinity of
the fullerene cage to the distortion of its geometry: ideal
structures with the symmetry Ih (С60) and D5h (C70) have
the highest electron affinity, and any deviations from the
perfect geometry decrease the withdrawing properties of
the core.
The elongation of the С—С bond in the metalꢀcoorꢀ
dinated fullerene over that in free fullerene affects subꢀ
* Complete Xꢀray diffraction data for the C60Pd(dppcym)2,
C
60Pd(dppr), and C70Pd(dppr) complexes will be published elseꢀ
where.

Electrochemistry of exohedral fullerene complexes
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 4, April, 2004
799
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This work was financially supported by the Russian
Foundation for Basic Research (Projects Nos. 01ꢀ03ꢀ
33147 and 03ꢀ03ꢀ32695) and the Federal Programs of the
Ministry of Science and Technologies (contracts of the
Presidium of the Russian Academy of Sciences No. 554ꢀ01
and the Division of Chemistry and Materials Science
No. 591ꢀ04).
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References
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Received April 28, 2003;
in revised form February 12, 2004