Photo-oxidation of bis[1,2-bis(diphenylphosphino)ferrocene]-palladium(0) in CCl4 induced by ferrocene to solvent charge transfer excitation

Article abstract of DOI:10.1016/S0022-328X(98)00411-2

The electronic spectrum of Pd⁰[(PPh₂C₅H₄)₂Fen]2 in CCl₄ shows an absorption at λ_max = 338 nm that is assigned to a charge transfer-to-solvent (CTTS) transition from the ferrocene moiety to CCl₄. The CTTS excitation leads to the formation of Pd^II[(PPh₂C₅H₄)₂Fe ^II]Cl₂. It is suggested that the irradiation induces initially the generation of Fe^III, which then oxidizes Pd⁰ by intramolecular electron transfer. Product formation takes place by a disproportionation of Pd^I.

Full text of DOI:10.1016/S0022-328X(98)00411-2

Journal of Organometallic Chemistry 559 (1998) 215–217
Preliminary communication
Photo-oxidation of
bis[1,2-bis(diphenylphosphino)ferrocene]-palladium(0) in CCl induced
4
by ferrocene to solvent charge transfer excitation
Horst Kunkely, Arnd Vogler *
Institut f u¨ r Anorganische Chemie, Uni6ersit a¨ t Regensburg, D-93040 Regensburg, Germany
Received 12 December 1997
Abstract
0
II
The electronic spectrum of Pd [(PPh C H ) Fe ] in CCl shows an absorption at umax=338 nm that is assigned to a charge
2
5
4 2
2
4
transfer-to-solvent (CTTS) transition from the ferrocene moiety to CCl . The CTTS excitation leads to the formation of
4
II
II
III
0
Pd [(PPh C H ) Fe ]Cl . It is suggested that the irradiation induces initially the generation of Fe , which then oxidizes Pd by
intramolecular electron transfer. Product formation takes place by a disproportionation of Pd . © 1998 Elsevier Science S.A. All
rights reserved.
2
5
4 2
2
I
Keywords: Charge transfer; Palladium complexes; Ferrocene complexes; Photochemistry
1
. Introduction
e.g. molecular electronics and artificial photosynthesis.
A suitable polynuclear complex that could serve as a
simple model is represented by the structure [9]:
Polynuclear heterometallic transition metal com-
plexes have attracted much attention in recent years.
This interest is related to the importance of these
compounds in various fields, such as biochemistry and
catalysis. These functions depend on several effects,
including electronic coupling of the metal centers. Un-
der suitable conditions, the metal–metal interaction
generates an electronic metal-to-metal charge transfer
(
MMCT) transition that appears as an absorption in
In chlorinated solvents, ferrocene shows a photoac-
tive charge transfer-to-solvent (CTTS) absorption
the electronic spectrum [1–6]. MMCT excitation can
result in a photoredox reaction [4,5]. As another in-
triguing possibility, a photoredox process may take
place at one metal center but subsequent electron trans-
fer creates a change of the redox state at the second
metal. This type of process might be important for
applications in supramolecular photochemistry [7,8],
II
[10,11]. CTTS excitation leads to the oxidation of Fe
III
to Fe . If the second metal M is reducing, the photo-
oxidation may be followed by an electron transfer from
III
M to Fe . As a result, the ferrocene moiety would
finally remain chemically intact, while only the second
metal is oxidized. We explored this possibility and
0
II
selected the complex Pd [(PPh C H ) Fe ] [12] with
*
Corresponding author. Tel.: +49 941 9434485; fax: +49 941
2
5
4 2
2
II
9
434488.
Fe (C H PPh )
denoting
1,1%-bis(diphenylphos-
5
4
2 2
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00411-2

216
H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 559 (1998) 215–217
phine)ferrocene (BDPF) for the present study. This
choice was based on several previous observations. Our
target complex is easy to synthesize and should undergo
a facile oxidation to Pd(II). Moreover, our recent study
changes (Fig. 2) that result in a general growth of the
absorbance over the entire UV–vis spectral region. This
apparent absorption, which increases towards shorter
wavelengths, is caused by light scattering at colloidal
particles. The colloidal photoproduct is separated by
I
III
+
of {Re [(PPh C H ) Fe ](CO) Cl} has shown that Fe
2
5
4 2
3
and Re are sufficiently coupled to provide an efficient
centrifugation, and identified as Pd(BDPF)Cl
2
by its
path for electron transfer [13]. Accordingly,
UV spectrum in CCl (umax=336 nm, m=7200 and 276
4
0
Pd (BDPF) should be an excellent candidate for fer-
nm, o=41600) and IR spectrum of a solid sample (w¯
2
0
−1
rocene to R–Cl CTTS-induced photo-oxidation of Pd .
PdꢀCl: 260 and 248 cm ) [12]. Since the absorption
spectra of Pd(BDPF) and Pd(BDPF)Cl are dominated
2
2
by the bands of the BDPF ligand, they are quite
similar. Accordingly, the spectral variations during the
2
. Results
photolysis of Pd(BDPF) are not accompanied by the
2
0
appearance of new absorption features (Fig. 2). How-
The electronic spectrum of Pd (BDPF) in CH CN
2
3
ever, Pd(BDPF)Cl is less soluble than Pd(BDPF) , and
(
(
(
Fig. 1) shows an absorption maximum at u=450 nm
m=1200 M cm ) and a shoulder at u=270 nm
24000). In CCl , which has a solvent cut-off at uB270
2
2
−
1
−1
separates as a colloid from the homogenous solution.
The stoichiometry of this photoreaction is established
by the following procedure. The colloid of a partially
photolyzed solution is separated by centrifugation and
4
nm, the long-wavelength band now appears as a shoul-
der at 462 nm (m=1500), while a rather intense new
band shows up (Fig. 1) at umax=338 nm (9100).
Upon irradiation (Hanovia Xe/Hg 977 B-1 lamp) of
Pd (BDPF) in CCl with u \300 nm, a photolysis
takes place that does not depend on the presence of
oxygen. This photoreaction is associated with spectral
redissolved in CCl . This solution, as well as the clear
4
solution obtained from centrifuging, were spectropho-
tometrically analyzed for Pd(BDPF)Cl2 and Pd(B-
0
2
4
irr
DPF) , respectively. This determination yields a ratio of
2
1
:0.78 for Pd(BDPF) to Pd(BDPF)Cl . The progress of
2 2
the photolysis is monitored by measuring the increase
of the optical density at 700 nm. The determination of
the relative quantum yields for the formation of Pd(B-
DPF)Cl2 is based on this increase of the apparent
absorption at this wavelength. Monochromatic light
was obtained using a Schoeffel GM 250/1 high-intensity
monochromator. The absorbed light intensities were
determined by a Polytec pyroelectric radiometer, which
was calibrated by actinometry and equipped with a
RkP-345 detector. The quantum yields are strongly
dependent on the irradiating wavelength. While they
reach a maximum at u =333 nm (ƒ =1.0), they are
irr
rel
relatively small at u =313 nm (ƒ =0.5), 366 nm
irr
rel
(
0.17) and 405 nm (0.07).
−
5
Fig. 1. Electronic absorption spectra of 5.47×10
CH CN (—) and CCl4 (. . . . . .) at r.t., 1 cm cell.
M Pd(BDPF) in
2
3
3. Discussion
The electronic spectrum of Pd(BDPF) in CH CN
2
3
(
Fig. 1) shows long-wavelength absorptions that belong
only to the BDPF ligand. This conclusion is consistent
0
with the observation that Pd (PPh ) displays its
3
4
longest-wavelength band at umax=322 nm [14]. The
longest-wavelength absorption of Pd(BDPF) at umax=
2
4
50 nm is assigned to the lowest-energy ligand field
(
LF) transition of the ferrocene moiety [6,10,11]. Fer-
rocene (umax=440 nm) and many of its derivatives,
show this characteristic absorption band in the same
wavelength region. If Pd(BDPF) is dissolved in CCl , a
2
4
new intense absorption appears at umax=338 nm. In
analogy to ferrocene [10,11], this band is assigned to a
CTTS transition from the ferrocene moiety to the halo-
carbon solvent.
−
5
Fig. 2. Spectral changes during the photolysis of 2.74×10
M
Pd(BDPF)2 in CCl4 at r.t. after 0 (a), 2 (b), 4 (c) and 8 (d) min
irradiation time with uirr=333 nm, 1 cm cell.

H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 559 (1998) 215–217
217
CTTS excitation of ferrocene in chlorinated alkanes
leads to the oxidation of ferrocene to the ferrocenium
plexes and may be important for applications in
supramolecular photochemistry.
cation [10,11]:
hw
Fe Cp +RꢁCl“[Fe Cp ] Cl +R’
II
III
+
−
2
2
Acknowledgements
There is little doubt that the same type of CTTS
Support of this research by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemis-
chen Industrie is gratefully acknowledged.
excitation of Pd(BDPF) also induces the photo-oxida-
2
tion of a ferrocene moiety:
0
II
2 5 4 2 2
{
Pd [(PPh C H ) Fe ] }
hw
CCl “{Pd [(PPh C H ) Fe ][(PPh C H ) Fe ]}
III
II
+
+
+
References
4
0
2
5
4 2
2
5
4 2
−
’
Cl + CCl
3
[1] D. Brown, Mixed-Valence Compounds, Reidel, Dordrecht, 1980.
[
[
[
2] C. Creutz, Progr. Inorg. Chem. 30 (1983) 1.
3] N.S. Hush, Progr. Inorg. Chem. 8 (1967) 391.
4] A. Vogler, A.H. Osman, H. Kunkely, Coord. Chem. Rev. 64
(1985) 159.
5] A. Vogler, in: M.A. Fox, M. Chanon (Eds.), Photo-induced
Electron Transfer, Elsevier, Amsterdam, 1988, p. 179.
6] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Am-
sterdam, 1984.
7] V. Balzani, Supramolecular Photochemistry, Reidel, Dordrecht,
1987.
8] V. Balzani, F. Scandola, Supramolecular Photochemistry, Hor-
wood, Chichester, 1991.
However, the ferrocenium cation is not present in the
final photoproduct, as indicated by the absence of a
diagnostic absorption band at 620 nm [6,10,11,15].
Ferrocenium is a strong oxidant and is apparently able
[
[
[
[
0
to oxidize Pd :
0
III
II
+
{
Pd [(PPh C H ) Fe ][(PPh C H ) Fe ]}
2
5
4 2
2
5
4 2
I
II
+
−
“
{Pd [(PPh C H ) Fe ] } Cl
2 5 4 2 2
Product formation can now occur by a subsequent
I
[9] K.-S. Gan, T.S.A. Hor, in: A. Togni, T. Hayashi (Eds.), Fer-
rocenes, VHC, Weinheim, 1995, p. 3.
[10] G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry,
Academic, New York, 1979.
disproportionation of Pd :
I
+
−
2
[Pd (BDPF) ] Cl
2
0
II
“
Pd (BDPF) +Pd (BDPF)Cl +BDPF
[11] C. Gianotti, S. Gaspard, P. Krausz, in: M.A. Fox, M. Chanon
2
2
(
Eds.), Photo-induced Electron Transfer, Elsevier, Amsterdam,
In summary, the outer sphere photo-oxidation of the
1988, p. 200.
0
[
12] G.M. de Lima, C.A.L. Filgueiras, M.T.S. Giotto, P. Mascaren-
has, Transit. Met. Chem. 20 (1995) 380.
13] K. Oldenburg, A. Vogler, J. Organomet. Chem. 544 (1997) 101.
14] J.V. Caspar, J. Am. Chem. Soc. 107 (1985) 6718.
15] Y.S. Sohn, D.N. Hendrickson, H.B. Gray, J. Am. Chem. Soc. 93
(1971) 3603.
ferrocene moiety of Pd (BDPF) is followed by a facile
2
0
III
intramolecular electron transfer from Pd to Fe ,
which finally leads to Pd as a stable photoproduct.
[
[
[
II
This novel photoredox process represents a special type
of photo-induced electron transfer in polynuclear com-
.