Photolysis of tetraphenylcyclobutadiene palladium chloride induced by LMCT excitation

Article abstract of DOI:10.1016/j.inoche.2005.10.035

The complex tetraphenylcyclobutadiene palladium(IV) chloride shows a longest-wavelength absorption at λ_max = 530 nm which is assigned to a LMCT transition from the C₄Ph₄ dianion to Pd(IV). LMCT excitation leads to the release of Pd(CH₃CN) ₂Cl₂ and neutral C₄Ph₄ which dimerizes to octaphenylcyclooctatetraene.

Full text of DOI:10.1016/j.inoche.2005.10.035

Inorganic Chemistry Communications 9 (2006) 248–250
www.elsevier.com/locate/inoche
Photolysis of tetraphenylcyclobutadiene palladium chloride
induced by LMCT excitation
Horst Kunkely, Arnd Vogler *
Institut f u¨ r Anorganische Chemie, Universit a¨ t Regensburg, D-93040 Regensburg, Germany
Received 28 September 2005; accepted 6 October 2005
Available online 22 December 2005
Abstract
The complex tetraphenylcyclobutadiene palladium(IV) chloride shows a longest-wavelength absorption at kmax = 530 nm which is
assigned to a LMCT transition from the C Ph dianion to Pd(IV). LMCT excitation leads to the release of Pd(CH CN) Cl and neutral
Ph which dimerizes to octaphenylcyclooctatetraene.
2005 Elsevier B.V. All rights reserved.
4
4
3
2
2
C
4
4
ꢀ
Keywords: Electronic spectra; LMCT; Palladium; Cyclobutadiene complexes; Photochemistry
Cyclobutadiene complexes of transition metals represent
an important family of organometallic compounds [1].
Their electronic structure has attracted much attention
for many years. In a simplified description they can be
this possibility and selected the complex [(C Ph )PdCl ]
4 4 2 2
[1,4,5] for the present study.
Ph
2ꢀ
4
viewed as complexes which contain C H4 as ligand. The
cyclobutadienyl dianion is a 6p-electron aromatic ring sys-
tem which conforms to the H u¨ ckel rule (4n + 2). While
Ph
2
Ph
Cl
Cl
Cl
Ph
2
2
ꢀ
4
PdIV
PdIV
C H coordinates in a g fashion it uses its three p-elec-
4
4
Ph
tron pairs for bonding interactions with the metal. In this
sense it is a tridentate ligand. It is quite informative to com-
Ph
Cl
Ph
2
ꢀ
pare C
4
H4 with the other 6p-electron C H H u¨ ckel aro-
n
n
þ
ꢀ
matic ring systems C
5
H , C H and C
7
H
which also
Ph
5
6
6
7
coordinate to transition metals. The HOMO is always a
The electronic spectrum of [(C Ph )PdCl ] in CH CN
4
4
2 2
3
degenerate pair of p-orbitals with increasing energy from
þ
2ꢀ
(Fig. 1) shows absorptions at kmax = 530 (sh, e =
C H to C H . While some observations on the light sen-
7
7
4
4
ꢀ1
ꢀ1
7
20 M cm ), 387 (9500), 313 (19,900), 282 (sh, 14,500)
sitivity of cyclobutadiene complexes have been reported the
nature of their reactive excited states has been hardly dis-
and 232 (sh, 17,700) nm. These solutions are light sensitive.
Upon irradiation with visible light a photolysis takes place
which is accompanied by distinct spectral changes (Fig. 1).
The sharp isosbestic point at k = 268 nm indicates the
occurence of a rather clean reaction. The most striking fea-
ture is the evolution of a new absorption at kmax = 242 nm
which is caused by Pd(CH CN) Cl (kmax = 242 nm,
2ꢀ
4
cussed [2,3]. Since the HOMO of C H4 is located at rather
high energies it should play an important role also in the
excited state properties of suitable complexes. We explored
*
Corresponding author. Tel.: +49 941 943 4716; fax: +49 941 943 4488.
3
2
2
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
e = 23,800) [6] as photolysis product. While solutions of
1
387-7003/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.10.035

H. Kunkely, A. Vogler / Inorganic Chemistry Communications 9 (2006) 248–250
249
2
-
PdIV
L
Pd
L
L
L
L
L
2a1
b1u
2e
LMCT
ꢀ5
Fig. 1. Spectral changes during the photolysis of 6.93 · 10
M
[
(Ph )PdCl in CH CN at room temperature after 0 s (a), 5, 10, 20
4
C
4
2
]
2
3
eg
and 80 s (b) irradiation times with kirr > 340 nm (Osram HBO 200 W/2
lamp; Schott cut off WG 360), 1 cm cell.
[
(C Ph )PdCl ] in CH CN are not luminescent, the
4 4 2 2 3
1
a1
irradiation leads to the appearance of an emission at
kmax = 389 nm which shows an excitation spectrum with
kmax = 275, 315, 340 and 360 nm. This luminescence is
apparently caused by octaphenylcyclooctatetraene [7]. In
agreement with these observations the photolysis proceeds
according to a rather simple equation:
1e
a2u
½
ðC
!
4
Ph
4
ÞPdCl
2
ꢁ2 þ 4CH
3
CN
2PdðCH
3
CNÞ Cl
2
þ C
8
Ph
8
ð1Þ
2
IV
4 4 3
Fig. 2. MO diagram of (C H )Pd L complexes, adapted from Ref. [13].
The progress of the photolysis was monitored by mea-
2
ꢀ
suring the decrease of the optical density at 387 nm. The
quantum yield for the disappearance of [(C Ph )PdCl ]
2 2
4
4
ligand (Fig. 2). Although C H is an aromatic 6p-elec-
tron system it is not stable because the degenerate
4
4
was / = 0.08 at kirr = 436 nm.
The dimeric compound [(C Ph )PdCl ] consists of two
HOMO e is nonbonding and located at very high ener-
g
2ꢀ
gies. However, in complexes, C H is stabilized because
4
4
4
2 2
4
monomeric (C Ph )PdCl fragments. To a first approxima-
the e orbitals are lowered by bonding interaction with
4
4
3
g
tion the electronic features can be explained by the proper-
ties of the mononuclear components. The assignment of
two negative charges to the C Ph ligand leads to the con-
the metal [9–13]. According to the high oxidation state
of Pd(IV) the d orbitals are situated at relatively low
energies. In the complex fragment the frontier orbitals
4
4
2ꢀ
clusion that palladium exists in the oxidation state IV. It
4
4
g 3
are derived from the C H e and the PdL 2e metal d
6
follows that the metal has a d electron configuration which
orbitals. The HOMO and the LUMO are strongly delo-
2
4
ꢀ
IV
requires an octahedral coordination. Indeed, the
calized between C H
and Pd . However, in the
4
(
C Ph )PdCl fragment can be viewed as an octahedral
HOMO which is C H –Pd bonding, the ligand character
4 4
prevails while the LUMO which is C H –Pd antibonding
4 4
4
4
3
2ꢀ
complex since the C Ph ligand provides three p-electron
4
4
pairs for the occupation of three bonding sites at
palladium.
is dominated by the metal d-orbitals. In summary, the
electronic structure of C H PdCl can be easily described
4
4
3
IV
What are the low-energy transitions of (C Ph )Pd Cl3?
on the basis of the occupied MOs (Fig. 2). While the
degenerate HOMO and the lowest-energy MO (a2u of
C H ) represent the three p-electron pairs of the ligand
4
4
6
Palladium(IV) with a d configuration has available LF
states but at very high energies [8]. Moreover, the LF
absorptions (t2g ! e in O symmetry) are generally quite
4
4
2
ꢀ
g
h
4
C H , the three other occupied MOs (1a and 1e of
4
1
2
ꢀ
IV
weak. In contrast, a C Ph ! Pd LMCT transition is
PdL ) correspond to the t d-orbitals of an octahedral
3 2g
4
4
6
expected to occur at rather low energies since Pd(IV) is
d complex. The HOMO–LUMO transition can be for-
mally viewed as LMCT transition (Fig. 2) but in reality
the participating MOs are strongly mixed and the extent
of charge separation may be rather small. Irrespective of
2
ꢀ
strongly oxidizing [8] and C Ph4 is certainly a strong
4
reductant. Consistent with this assumption we assign the
longest-wavelength absorption at 530 nm to the
2
ꢀ
IV
2ꢀ
IV
C
4
Ph4 ! Pd LMCT transition of the complex.
IV
these details the C H ! Pd LMCT excitation is not
ꢀ
IV
4
4
4
2
Let us now have a closer look at the electronic struc-
ture of (C H )Pd L complexes with L = any simple
only associated with a charge shift from C H to Pd
4
but also with a strong destabilization of the C H –Pd
4 4
4
4
3

250
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 9 (2006) 248–250
bond. Consequently, LMCT excitation of [(C Ph )PdCl ]
2 2
of the LF type. In agreement with this assumption the irra-
diation of (C H )Fe(CO) does not lead to the release of
4
4
leads to the release of the neutral C Ph molecule which,
4
4
4
4
3
owing to its antiaromatic nature, is not stable. It is well
known to dimerize to octaphenylcyclooctatetraene [14]
as also observed in the present study.
C H but to the ejection of a CO ligand in the primary pho-
4 4
tochemical step [2].
How do our results compare with those which have
previously obtained in the photochemical studies of
References
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C H )Fe(CO) [2] and (C Ph )NiBr [3]. In distinction
4 4 3 4 4 2
[
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[
[
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4
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4
2
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[
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4
4
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II
IV
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[
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(
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