Excited state properties of [Pd0(NHC)(quinone)]2 with NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and quinone = 1,4-naphthoquinone

Article abstract of DOI:10.1016/j.jorganchem.2005.07.114

The carbene complex [Pd⁰(NHC)(quinone)]₂with NHC = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene and quinone = 1,4-naphthoquinone shows two long-wavelength absorptions at 312 and 399 nm which are assigned to (NHC→quinone) LLCT and (Pd⁰ → quinone) MLCT transitions. The MLCT state is not reactive, but emissive (λ_max = 564 nm at 77 K). At r.t., the complex undergoes a photoredox decomposition which is initiated by the LLCT state.

Full text of DOI:10.1016/j.jorganchem.2005.07.114

Journal of Organometallic Chemistry 690 (2005) 6051–6053
www.elsevier.com/locate/jorganchem
Excited state properties of [Pd0(NHC)(quinone)]2 with
NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and
quinone = 1,4-naphthoquinone
*
Horst Kunkely, Arnd Vogler
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany
Received 31 May 2005; accepted 29 July 2005
Available online 19 September 2005
Abstract
The carbene complex [Pd0(NHC)(quinone)]2with NHC = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene and quinone = 1,4-naph-
thoquinone shows two long-wavelength absorptions at 312 and 399 nm which are assigned to (NHC!quinone) LLCT and (Pd0 ! qui-
none) MLCT transitions. The MLCT state is not reactive, but emissive (kmax = 564 nm at 77 K). At r.t., the complex undergoes a
photoredox decomposition which is initiated by the LLCT state.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Electronic spectra; Charge transfer; Photochemistry; Photoluminescence; Palladium complexes; Carbene complexes
1. Introduction
the reactivity of NHC complexes in general. Our present
investigation of [Pd0(NHC)(quinone)]2 [6] with NHC =
1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene and qui-
none = 1,4-naphthoquinone is facilitated by the reasonable
thermal stability of this compound under ambient condi-
tions.
N-heterocyclic carbenes (NHCs) have attracted much
attention in recent years [1–4]. These compounds exist as
stable species and function as ligands in coordination
chemistry. A variety of such complexes has been prepared,
characterized and applied in organometallic catalysis. Sur-
prisingly, very little is known about the photochemistry
and photophysics of transition metal complexes with
NHC ligands [5]. Since the light-sensitivity of these com-
pounds may interfere with their role in stoichiometric or
catalytic reactions a deeper insight in their excited state
behavior is of considerable importance. Of course, this sub-
ject is also rather interesting in its own right. Our present
study is an extension of a previous investigation of
Ni0(NHC)(CO)2 and TiIV(NHC)2Cl4 [5]. The new observa-
tions show that additional ligands which are introduced in
NHC complexes may modify or even completely change
the excited state properties of such complexes. While this
is not unexpected such studies are necessary to understand
O
Me3Ph
Me3Ph
N
O
N
Pd0
O
Pd0
O
N
N
Me3Ph
Me3Ph
2. Experimental
*
1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene(1,4-
naphthoquinone) palladium(0) dimer (ABCR GmbH&Co)
Corresponding author. Tel.: +49 941 943 4716; fax: +49 941 943 4488.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.114

6052
H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 690 (2005) 6051–6053
and 1,4-dihydroxynaphthalene (Acros Organics) were com-
mercially available and used without further purification.
All solvents used for spectroscopic measurements were of
spectrograde quality (‘‘Uvasol’’) from Merck and saturated
with argon.
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 18 nm) with additional
Schott cutoff filters to avoid short-wavelength and second
order irradiation. In all cases the light beam was focused
on a stirrable photolysis cell by a quartz lens. The photo-
lyses were carried out in acetonitrile under argon in 1 cm
spectrophotometer cells at room temperature under argon.
Progress of the photolyses was monitored by UV–Vis spec-
trophotometry. For quantum yield determinations the
complex concentrations 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 determined
by a Polytec pyroelectric radiometer, which was calibrated
by ferrioxalate actinometry and equipped with an RkP-345
detector.
Fig. 2. Spectral changes during the photolysis of 3.93 · 10ꢀ5
[Pd0(NHC)(quinone)]2 in CH3CN under argon at room temperature after
0 (a), 20 and 80 s (b) irradiation times with kirr = 405 nm (Hanovia Xe/Hg
977 B-1 lamp), 1-cm cell.
yellow photoluminescence (Fig. 1) at kmax = 564 nm. The
excitation spectrum roughly matches the absorption spec-
trum. At r.t. [Pd0(NHC)(quinone)]2 is not emissive, but
light sensitive. The photolysis in wet CH3CN leads to a
bleaching (Fig. 2) and at later stages to the appearance of
palladium black. The colloidal metal causes an apparent
absorption by light scattering which extends over the entire
spectrum and increases towards shorter wavelength [7].
Naphthohydroquinone (or 1,4-dihydroxynaphthalene)
was formed as a further photoproduct. It was identified
by its characteristic fluorescence (kmax = 405 nm) which in-
creased its intensity with the duration of the irradiation.
The progress of the photolysis was monitored by measur-
ing the decrease of the optical density at 399 nm taking into
account the residual absorption of the photolysis products.
The complex [Pd0(NHC)(quinone)]2 disappeared with
/ = 0.12 at kirr = 313 nm and / = 0.02 at kmax = 436 nm.
Absorption spectra were measured with a Varian Cary
50 or a Uvikon 860 spectrophotometer. Emission and exci-
tation spectra were recorded on a Hitachi 850 fluorescence
spectrometer equipped with a Hamamatsu 928 photomulti-
plier for measurements up to 900 nm. The luminescence
spectra were corrected for monochromator and photomul-
tiplier efficiency variations.
3. Results
The electronic spectrum of [Pd0(NHC)(quinone)]2 in
CH3CN (Fig. 1) shows absorptions at
kmax = 399
(e = 10800 Mꢀ1 cmꢀ1), 312 (sh, 18500), 281 (sh, 32300),
245 (sh, 89600) and 206 (sh, 156500) nm. At 77 K in an
ethanol glass the complex displays a weak, but distinct
4. Discussion
The dimeric compound [Pd0(NHC)(quinone)]2 consists
of two monomeric Pd(0) complex fragments which contain
a carbene ligand and two quinones [6]. They provide the
bridges for the dimeric structure. To a first approximation,
the electronic features can be explained by the properties of
the mononuclear components. Pd(0) has a closed shell (d10)
and LF states are thus not available. Owing to this reduc-
ing character, Pd(0) can function as CT donor. Carbenes of
the NHC type are strong r donors and accordingly also
CT donors of considerable strength [5]. Simultaneously,
they are weak p and CT acceptors [5]. Quinones are strong
oxidants and powerful CT acceptors [8].
From these considerations, it follows that [Pd0(NHC)-
(quinone)]2 should be characterized by two low-energy
electronic transitions: Pd(0) to quinone MLCT (metal-to-
ligand charge transfer) and NHC to quinone LLCT
(ligand-to-ligand charge transfer). In agreement with the
Fig. 1. Electronic absorption (a) and emission (e) spectrum of
[Pd0(NHC)(quinone)]2 under argon. Absorption: 1.57 · 10ꢀ5 M in
CH3CN at r.t. 1-cm cell. Emission: 2.25 · 10ꢀ5 M in EtOH at 77 K,
kexc = 380 nm, intensity in arbitrary units.

H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 690 (2005) 6051–6053
6053
spectral and photochemical properties of the complex, we
suggest that the longest-wavelength absorption at
lic ketone NHCO might be generated but further fragmen-
tations could take place. Irrespective of this detail the
photolysis is initiated by CT from NHC to the quinone.
The photoreactivity can thus be attributed to the lowest-
energy LLCT state. In agreement with this conclusion the
photochemical quantum yield is higher upon irradiation
of the LLCT band (/ = 0.12 at kirr = 313 nm) compared
to that for the MLCT excitation (/ = 0.02 at
k
max = 399 nm (Fig. 1) is of the MLCT type while the next
band at 312 nm (sh) belongs to the LLCT transition. Both
absorptions are relatively intense and the corresponding
transitions are thus spin-allowed. At 500 nm (Fig. 1) a
weak inflection can be recognized which probably belongs
to a spin-forbidden (singlet–triplet) MLCT transition.
Generally, MLCT states are frequently luminescent [9]
and only occasionally reactive [10,11]. We assume that
the emission of [Pd0(NHC)(quinone)]2 at 77 K originates
from the lowest-energy MLCT triplet. In agreement with
this suggestion, there is some overlap of the emission with
the long-wavelength tail of the absorption which was as-
signed to the spin-forbidden MLCT transition.
k
irr = 436 nm). The residual photosensitivity which is ob-
served on MLCT excitation can be explained, at least par-
tially, by a thermally activated population of the reactive
LLCT state. As a further consequence the phosphorescence
of [Pd0(NHC)(quinone)]2 is quenched at r.t.
At r.t. the phosphorescence of [Pd0(NHC)(quinone)]2 is
absent while the compound is photoreactive. The photo-
chemical observations are consistent with the following
simple equation:
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[Pd0(NHC)(quinone)]2 + 2H2O
!2Pd0 þ 2NHCO þ 2hydroquinone
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ð1Þ
The quinone undergoes a two-electron reduction and pro-
tonation to hydroquinone. Since Pd(0) does not donate
these electrons they should be provided by NHC. A variety
of oxidation products are conceivable, but within the scope
of the present study we did not pursue any further analysis.
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