Excited state properties of dimeric π-allylpalladium(II) chloride. Photoreduction of palladium induced by ligand-to-metal charge transfer excitation

Article abstract of DOI:10.1016/S0020-1693(02)01212-4

The longest-wavelength absorption of [(π-allyl)Pd^IICl]₂ at λ_max=318 nm is assigned to an allyl^-→Pd^II ligand-to-metal charge transfer (LMCT) transition. In the primary photochemical step LMCT excitation leads to the formation of an allyl radical and Pd^ICl, which in acetonitrile undergoes a disproportionation to metallic palladium and palladium(II). Pd^II(CH₃CN)₂Cl₂ is formed with φ=0.008 at λ_irr=366 nm.

Full text of DOI:10.1016/S0020-1693(02)01212-4

Inorganica Chimica Acta 344 (2003) 262Á264
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www.elsevier.com/locate/ica
Note
Excited state properties of dimeric p-allylpalladium(II) chloride.
Photoreduction of palladium induced by ligand-to-metal charge
transfer excitation
Horst Kunkely, Arnd Vogler *
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universitaetsstrasse 31, D-93040 Regensburg, Germany
Received 2 July 2002; accepted 8 August 2002
Abstract
The longest-wavelength absorption of [(p-allyl)PdIICl]2 at lmax
transfer (LMCT) transition. In the primary photochemical step LMCT excitation leads to the formation of an allyl radical and
ꢀ
/
318 nm is assigned to an allylꢁ 0
/
PdII ligand-to-metal charge
PdICl, which in acetonitrile undergoes a disproportionation to metallic palladium and palladium(II). PdII(CH3CN)2Cl2 is formed
with fꢀ/0.008 at lirr
ꢀ366 nm.
/
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: UV spectra; Photochemistry; Charge transfer; Palladium complexes; Allyl complexes
1. Introduction
10], but a relationship to photochemical properties has
not been established. Moreover, such correlations are
now facilitated by extensive theoretical work on the
electronic structure of palladium allyl complexes. These
calculations have been carried out in recent years in
order to understand the reactivity in the electronic
Palladium(II) complexes with p-allyl ligands play an
important role in stoichiometric and catalytic transfor-
mations of organic compounds [1Á4]. In a key step
/
nucleophiles attack the allyl ligand. In contrast to
numerous studies of thermal reactions, relatively little
is known about the photoreactivity of Pd(II) allyl
ground state [11Á16].
/
complexes [5Á7]. The present work was undertaken to
/
characterize the reactive excited states of such com-
pounds. For this purpose we selected the complex [(p-
allyl)PdIICl]2 as a suitable subject.
2. Experimental
2.1. Materials
All solvents used for spectroscopic measurements
were of spectrograde quality and saturated with Ar.
The complex [(p-allyl)PdIICl]2 was commercially avail-
able from Strem and used without further purification.
Some important observations on the photochemistry of
this compound have already been reported, but the
origin of the light sensitivity has not been considered in
2.2. Instrumentation
these studies [5Á
absorption spectra of [(allyl)PdCl]2 and other Pd(II)
allyl complexes have been recorded and discussed [8Á
/
7]. On the other hand, the electronic
Absorption and emission/excitation spectra were
measured with a Uvikon 860 absorption spectrometer
and a Hitachi 850 fluorescence spectrometer equipped
with a Hamamatsu 928 photomultiplier for measure-
ments up to 900 nm. The luminescence spectra were
corrected for monochromator and photomultiplier effi-
/
* Corresponding author. Tel.: ꢂ
4488
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
/
49-941-943 4485; fax: ꢂ49-941-943
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 2 1 2 - 4

H. Kunkely, A. Vogler / Inorganica Chimica Acta 344 (2003) 262Á
/264
263
ciency variations. The light source used for irradiation
was an Osram HBO 200 W/2 or a Hanovia Xe/Hg 977
B-1 (1 kW) lamp. Monochromatic light was obtained
using Schott PIL/IL interference filters or a Schoeffel
GM/1 high-intensity monochromator (band width 23
nm) with additional Schott cutoff filters to avoid short-
wavelength and second order irradiation. In all cases the
light beam was focused on a thermostated photolysis
cell by a quartz lens.
occur, since the binuclear complex could be cleaved to
yield (allyl)Pd(solvent)Cl. In the solid state or in
solution, [(allyl)PdCl]2 does not display any lumines-
cence at room temperature or 77 K.
Solutions of [(allyl)PdCl]2 in CH3CN are thermally
quite stable, but light-sensitive. The photolysis is
accompanied by spectral changes which indicate the
formation of Pd(CH3CN)2Cl2. This complex exhibits
strong absorptions at lmax
ꢀ242 nm (23 800) and 211
/
(12 600). Moreover, colloidal palladium is formed in
agreement with previous observations [5,7]. The colloi-
dal particles cause an apparent absorption by light
scattering, which extends over the entire spectrum (Fig.
1) and increases towards shorter wavelengths. The
colloidal metal could be removed by centrifugation. It
was insoluble in hydrochloric acid but dissolved in nitric
acid. It was definitely identified by the violet Pd(II)
2.3. Photolyses
The photolyses were carried out in solutions of
CH3CN in 1-cm spectrophotometer cells at room
temperature under Ar. The progress of the photolyses
was monitored by UVÁVis spectrophotometry. For
/
quantum yield determinations the complex concentra-
tions were such as to have essentially complete light
absorption. The total amount of photolysis was limited
to less than 5% to avoid light absorption by the
photoproduct. Absorbed light intensities were deter-
mined by a Polytec pyroelectric radiometer which was
calibrated by ferrioxalate actinometry and equipped
with an RkP-345 detector.
complex (lmax
ꢀ370 and 550 nm) that is formed with 2-
/
nitroso-1-naphthol [17]. Progress of the photolysis was
monitored by measuring the increase of the absorbance
at 242 nm. At this wavelength the extinction coefficients
of [(allyl)PdCl]2 and Pd(CH3CN)2Cl2 are oꢀ7900 and
/
23 800, respectively. Taking into account the residual
absorptions by other photoproducts the change of
absorbance at 242 nm yields a stoichiometric ratio for
the conversion of [(allyl)PdCl]2 to Pd(CH3CN)2Cl2 of
approximately 1:1. The quantum yield for the formation
3. Results
of Pd(CH3CN)2Cl2 is fꢀ/0.008 at lirr
ꢀ366 nm and
/
0.006 at 313 nm.
The electronic spectrum of [(allyl)PdCl]2 in CH3CN
(Fig. 1) shows absorptions at lmax
ꢀ
/
318 nm (sh, oꢀ
/
1600 Mꢁ1 cmꢁ1), 285 (sh, 1500), 243 (sh, 6800), 224
(sh, 11 500) and 207 (17 700). The spectrum of this
4. Discussion
complex has been recorded before [8Á10], but with some
/
The binuclear compound [(allyl)PdIICl]2 is composed
of two Pd(II) complex moieties with d8 metal centers
which are held together by bridging chloride ligands. In
a simplified version the dimer can be viewed as contain-
ing two separate [(allyl)PdCl2]ꢁ complexes. While the
frontier orbitals of many other Pd(II) complexes are d
orbitals [18], the coordination of the allyl anion strongly
deviations from the present results. The long-wavelength
bands around 300 nm are only slightly dependent on the
solvent polarity (e.g. ethanol, CHCl3). In strongly
coordinating solvents minor spectral changes may
modifies the electronic structure [11Á16]. The allyl anion
/
is characterized by an occupied pb, an occupied pn and
an empty p* orbital (Fig. 2). The interaction of the bent
PdIICl2 (or generally PdIIL2) fragment with the allyl
anion is dominated by the overlap of the empty dxz
orbital of Pd(II) with the occupied pn allyl orbital, which
have comparable energies. It is assumed that this
interaction generates the bonding HOMO and the
antibonding LUMO of the complex (allyl)PdIIL2 (1aƒ
and 2aƒ, respectively, in Cs symmetry). Accordingly, the
lowest-energy transition from the HOMO to the LUMO
takes place between two orbitals which are strongly
mixed between the metal and the allyl ligand. This is
certainly not a typical dd (or ligand field, LF) transition.
If the complex is assumed to contain Pd(II), then this
Fig. 1. Spectral changes during the photolysis of 6.1ꢃ
/
10ꢁ5 M [(p-
allyl)PdIICl]2 in CH3CN at room temperature after 0 min (a), 0.5, 1
and 2 min (d) irradiation times with lirr
2 lamp), 1-cm cell.
ꢀ/320 nm (Osram HBO 200 W/
1aƒ0
/
2aƒ transition is formally of the allylꢁ 0
/
PdII

264
H. Kunkely, A. Vogler / Inorganica Chimica Acta 344 (2003) 262Á264
/
The quantum yield dependence of the photolysis sup-
ports the assumption that the longest-wavelength band
of [(allyl)PdCl]2 at lmax
ꢀ/318 nm belongs indeed to the
photoactive LMCT (or 1aƒ0
/
2aƒ, Fig. 2) transition.
In summary, it is concluded that in analogy to the
cyclopentadienyl anion [22] the allyl anion is a ligand
with CT donor character. Accordingly, allyl complexes
of oxidizing metals are generally expected to show long-
wavelength allylꢁ 0
/
M LMCT absorptions. However,
owing to extensive covalent bonding within the M-allyl
moiety, the LMCT assignment is a formal label since the
real charge redistribution may be rather small.
5. Supplementary material
Fig. 2. Qualitative MO scheme for (p-allyl)PdIIL2 complexes (Cs
symmetry).
The material is available from the authors on request.
ligand-to-metal charge transfer (LMCT) type, although
the actual CT contribution may be rather small. In line
with this model (Fig. 2), we assign the longest-wave-
Acknowledgements
Support of this research by the Fonds der Chemischen
Industrie is gratefully acknowledged.
length absorption of [(allyl)PdCl]2 at lmax
ꢀ318 nm to
/
this LMCT transition. The small CT character is
reflected by the observation that this band is subject to
only the slightest solvent-dependent shift. In this context
it is of interest that this absorption has been previously
assigned to an unspecified allyl-Pd transition that
remains unchanged on replacement of the halogen and
does not appear in the spectra of other Pd(II) halide
complexes without allyl ligands [9]. Moreover, the
photoreactivity of [(allyl)PdCl]2 can be easily attributed
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hn=LMCT allyl radicalꢂPdICl
(1)
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12[(allyl)PdCl]2
0
2PdIClꢂ2CH3CN 0 Pd0 ꢂPdII(CH3CN)2Cl2
2 allyl radicals 0 hexa-1; 5-diene
(2)
(3)