Electronic spectrum and photoreactivity of dichloro(1,5-cyclooctadiene)-palladium(II)

Article abstract of DOI:10.1016/S0022-328X(98)00483-5

The complex Pd^II(1,5-COD)Cl₂ (1,5-COD = 1,5-cyclooctadiene) shows a long-wavelength ligand-to-metal charge transfer (LMCT) absorption at λ_max = 342 nm. It is suggested that LMCT excitation generates a COD radical cation and Pd^ICl₂^- in the primary photochemical step. In acetonitrile, a back-electron transfer takes place that leads to the transformation of 1,5-COD to its 1,3-isomer and a solvated Pd(II) complex. In ethanol, the COD radical cation and Pd^I are reduced by the solvent, which is oxidized to acetaldehyde. Metallic palladium is formed as a colloid.

Full text of DOI:10.1016/S0022-328X(98)00483-5

Journal of Organometallic Chemistry 559 (1998) 223–225
Preliminary communication
Electronic spectrum and photoreactivity of
dichloro(1,5-cyclooctadiene)-palladium(II)
Horst Kunkely, Arnd Vogler *
Institut f u¨ r Anorganische Chemie, Uni6ersit a¨ t Regensburg, D-93040 Regensburg, Germany
Received 8 January 1998
Abstract
The complex Pd (1,5-COD)Cl (1,5-COD=1,5-cyclooctadiene) shows a long-wavelength ligand-to-metal charge transfer
II
2
I
−
(
LMCT) absorption at umax=342 nm. It is suggested that LMCT excitation generates a COD radical cation and Pd Cl2 in the
primary photochemical step. In acetonitrile, a back-electron transfer takes place that leads to the transformation of 1,5-COD to
I
its 1,3-isomer and a solvated Pd(II) complex. In ethanol, the COD radical cation and Pd are reduced by the solvent, which is
oxidized to acetaldehyde. Metallic palladium is formed as a colloid. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Absorption spectra; Palladium complexes; Photochemistry
searched for suitable complexes and decided to study
the compound Pd (1,5-COD)Cl2 (1,5-COD=1,5-
1. Introduction
II
cyclooctadiene).
Olefin complexes play an important role in
Palladium(II) is only weakly reducing, but also oxi-
dizing. It can be easily reduced to metallic palladium.
In fact, the ethylene oxidation to acetaldehyde by Pd
organometallic chemistry. A variety of such complexes
is light sensitive [1]. Photo-induced substitutions, iso-
merizations and oxidative additions of olefin ligands
have been observed. Generally, these photoreactions
can be related to electronic transitions that require the
participation of the olefin. Frequently, the reactive ex-
cited states are of the metal“y* (olefin) metal-to-lig-
and charge transfer (MLCT) and yy* (olefin)
intraligand (IL) type. In this context, it is quite surpris-
ing that olefin“metal ligand-to-metal charge transfer
2
+
in aqueous solution forms the basis of the Wacker
process [3]. However, the mechanism of this olefin
oxidation is rather complicated and does not simply
involve electron transfer from ethylene to Pd(II). Nev-
ertheless, palladium(II) olefin complexes are promising
candidates for photoredox reactions initiated by LMCT
excitation. Mono-olefin complexes are quite unstable
and reactive and thus, less suitable for our purpose. On
(
LMCT) transitions that could initiate olefin oxidation
the contrary, Pd(1,5-COD)Cl is rather stable and easily
2
and metal reduction have been largely ignored, al-
though olefins are well-known to be thermally oxidized
by many metal complexes [2]. Partly, this lack of inter-
est is related to the fact that the co-ordination of olefins
requires metals in low oxidation states that are usually
reducing and not oxidizing. In order to fill this gap we
accessible [4–6].
2
. Results
The electronic spectrum of Pd(1,5-COD)Cl2 in
*
Corresponding author. Tel.: +49 941 9434488.
CH CN (Fig. 1) shows absorptions at umax=400 (m=
3
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00483-5

224
H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 559 (1998) 223–225
3
−1
−1
6
60 dm M cm ), 342 (1930), 298 (3200), 273
(
4650), 240 (sh, 7200) and 219 nm (16200). In solution
of ethanol, the maxima are blurred. The complex does
not emit at r.t. or at 77 K, but solutions are light
sensitive.
The photolysis of Pd(1,5-COD)Cl in ethanol is ac-
2
companied by spectral variations (Fig. 2) that show an
increase of the optical density over the entire visible
absorption spectrum. This increase is associated with a
brown coloration, which indicates the formation of
colloidal palladium [7]. Moreover, the irradiation leads
to the evolution of a new band that appears as a
shoulder at around 240 nm (Fig. 2). This absorption is
attributed to 1,3-cyclooctadiene (umax=238 nm, m=
−
4
Fig. 2. Spectral changes during the photolysis of 2.07×10
M
II
Pd (1,5-COD)Cl in EtOH at r.t. after 0 (a), 4 (b) and 16 (c) min
irradiation time with white light (Osram HBO 100 W/2); 1 cm cell.
2
5900). Protons and acetaldehyde, which is identified as
an addition product of 4-hydroxydiphenyl (umax=560
nm) [8], are further photoproducts. The light-sensitivity
for several other Pd(II) complexes [9] including
Pd(en)Cl2 (umax=374 nm, m=292) [10] in the same
wavelength region. The higher intensity of the 400 nm
band of Pd(1,5-COD)Cl2 is certainly caused by the
strong overlap with the 342 nm absorption. This latter
band is too intense (m=1930) for a LF transition. An
IL (COD) transitions can be also ruled out since free
1,5-COD only absorbs at a very short wavelength
(umax=198 nm). As the next possibility, a MLCT as-
signment must be considered. Various other 1,5-COD
complexes of metals in low oxidation states display
low-energy M“y* (COD) MLCT bands [11–13]. For
example, Pt(1,5-COD)Cl2 shows this absorption at
umax=256 nm [14]. However, a comparison of Pt(-
bipy)Cl and Pd(bipy)Cl has shown that the MLCT
of Pd(1,5-COD)Cl extends over the whole absorption
2
spectrum, including the 342 nm band.
If the photolysis is carried out in acetonitrile, only
traces of elemental palladium are formed. At a longer
wavelength (\280 nm), all absorptions of Pd(1,5-
COD)Cl decrease in intensity, but at shorter wave-
2
lengths, the appearance of the 240 nm band is again
indicative of the formation of 1,3-cyclooctadiene (Fig.
3). The photoreaction is not influenced by oxygen and
also proceeds under argon. The photolysis is monitored
by measuring the decrease of the optical density at 342
nm. The quantum yields for the disappearance of
Pd(1,5-COD)Cl are ƒ=0.08 at u =366 nm and ƒ=
2
irr
0.03 at umax=405 nm.
2
2
(
M“bipy) band of the palladium complex (umax=318
nm, in water) occurs at shorter wavelength than that of
the platinum compound (umax=340 nm, in water) in
agreement with the expectation [15]. Accordingly, a
3. Discussion
MLCT band of Pd(1,5-COD)Cl should appear below
The electronic spectrum of Pd(1,5-COD)Cl (Fig. 1)
2
2
2
56 nm.
shows a rather complicated pattern, but the two
longest-wavelength bands at 400 and 342 nm can be
assigned with some confidence. It is suggested that the
band at 400 nm (m=660) belongs to the lowest-energy
spin-allowed LF (ligand field) transition, which appears
As the only remaining alternative, we assign the 342
nm band of Pd(1,5-COD)Cl
from y-bonding orbitals of COD to Pd(II). Since
to a LMCT transition
2
−
4
Fig. 3. Spectral changes during the photolysis of 4.36×10
M
−
4
II
II
Fig. 1. Electronic absorption spectrum of 1.46×10
M Pd (1,5-
Pd (1,5-COD)Cl in CH CN at r.t. after 0 (a), 20 (b), 40 (c) and 100
2 3
COD)Cl2 at r.t. in acetonitrile; 1 cm cell.
(d) s irradiation time with white light; 1 cm cell.

H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 559 (1998) 223–225
225
hw
Pd (1,5−COD)Cl +C H OH“Pd +1,3−COD
II
0
olefins are generally reducing, and Pd(II) is oxidizing,
such LMCT absorptions should occur at relatively long
wavelength in analogy to other Pd(II) complexes with
reducing ligands [9]. For example, the lowest-energy
2
2
5
+
2HCl+CH CHO
3
In summary, it should be emphasized that although
2
−
the observation of a reactive LMCT state involving an
olefin ligand is rather new, it is not surprising in view of
the reducing properties of olefins.
LMCT band of [Pd(N ) ]
appears at umax=324 nm
m=17700) [7]. Finally, the LMCT assignment of the
42 nm band of Pd(1,5-COD)Cl is also consistent with
3
4
(
3
2
the photochemical properties of this complex.
It is assumed that the photolysis of Pd(1,5-COD)Cl2
is initiated by COD-Pd(II) LMCT excitation. In the
primary photochemical step a radical pair should be
formed:
Acknowledgements
Financial support for this research by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemis-
chen Industrie is gratefully acknowledged.
hw
Pd (1,5−COD)Cl “COD Pd Cl2
II
+
’
I
−
2
It may undergo recombination or cage escape. In
acetonitrile, solution back-electron transfer leads to
product formation. The reduction of the COD radical
cation is apparently associated with the formation of
the 1,3-COD isomer. In this context it is quite interest-
ing that the photochemical rearrangement of 1,5-COD
to 1,3-COD is also catalyzed by Rh(I), although the
mechanism of this reaction is not quite clear [1,16].
References
[
[
1] G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry,
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Weinheim, 1996, p. 374.
I
−
Finally, the reoxidation of the Pd Cl fragment to
2
[
[
4] D. Drew, J.R. Doyle, Inorg. Synth. 28 (1990) 346.
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(Ed.), Handbuch der PraparativenAnorganischen Chemie, Ferdi-
nand Enke, Stuttgart, 1981, p. 1904.
3
2
2
ble photoproducts are not expected to absorb strongly,
the spectral variations during the photolysis of Pd(1,5-
COD)Cl in CH CN are not accompanied by the ap-
[
7] A. Vogler, C. Quett, H. Kunkely, Ber. Bunsenges. Phys. Chem.
2 (1988) 1486.
2
3
9
pearance of new long-wavelength bands (Fig. 3). The
overall reaction is assumed to proceed according to the
equation:
[
8] M.C. Bowman, M. Beroza, F. Acree Jr., Anal. Chem. 33 (1961)
1053.
[9] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Am-
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[10] D.S. Martin, G.A. Robbins, R.M. Rush, Inorg. Chem. 19 (1980)
3
hw/CH CN
II
II
Pd (1,5−COD)Cl
“
Pd (CH CN) Cl +1,3
3 2 2
2
1
705.
−
COD
[11] A.J. Lees, Chem. Rev. 87 (1987) 711.
[
12] R.A. Epstein, G.L. Geoffroy, M.E. Keeney, W.R. Mason, Inorg.
Chem. 18 (1979) 478.
At this point it should be mentioned that photosub-
[
[
[
13] W.A. Fordyce, G.A. Crosby, Inorg. Chem. 21 (1982) 1023.
14] H. Kunkely, A. Vogler, J. Organomet. Chem. in press.
15] P.M. Gidney, R.D. Gillard, B.T. Heaton, J. Chem. Soc. Dalton
Trans. (1973) 132.
stitutions initiated by LMCT excitation are quite com-
mon reactions in inorganic photochemistry. In
particular, Co(III) ammine complexes show this type of
LMCT reactivity [17].
[
[
16] (a) R. Srinivasan, J. Am. Chem. Soc. 86 (1964) 3318. (b) R.G.
Salomon, El Sanadi, J. Am. Chem. Soc. 97 (1975) 6214.
17] A. Vogler, H. Kunkely, in: K. Kalyanasundaram, G. Gratzel
In ethanol the photolysis of Pd(1,5-COD)Cl takes a
2
different course. The solvent is apparently able to re-
duce the primary radical pair COD’ Pd Cl . Product
formation takes place according to the equation:
(
Eds.), Photosensitization and Photocatalysis Using Inorganic
+
I
−
2
and Organometallic Compounds, Kluwer, Dordrecht, 1993, p.
71.
.