Photochemistry of trichloro(phenylazo)bis(triphenylphosphine)ruthenium(II) induced by Ru to phenyldiazonium MLCT excitation

Article abstract of DOI:10.1016/j.inoche.2005.10.002

The complex trichloro(phenylazo)bis(triphenylphosphine)ruthenium(II) shows a longest-wavelength absorption at λ_max = 520 nm which is assigned to a Ru(II) → phenyldiazonium⁺ MLCT transition. MLCT excitation leads to the oxidation of the metal and reduction of the diazonium cation which decays to nitrogen and phenyl radical.

Full text of DOI:10.1016/j.inoche.2005.10.002

Inorganic Chemistry Communications 8 (2005) 1185–1186
www.elsevier.com/locate/inoche
Photochemistry of
trichloro(phenylazo)bis(triphenylphosphine)ruthenium(II)
induced by Ru to phenyldiazonium MLCT excitation
*
Horst Kunkely, Arnd Vogler
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany
Received 23 June 2005; accepted 6 October 2005
Abstract
The complex trichloro(phenylazo)bis(triphenylphosphine)ruthenium(II) shows a longest-wavelength absorption at kmax = 520 nm
which is assigned to a Ru(II) ! phenyldiazonium+ MLCT transition. MLCT excitation leads to the oxidation of the metal and reduction
of the diazonium cation which decays to nitrogen and phenyl radical.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Electronic spectra; MLCT; Photochemistry; Ruthenium; Phenyldiazonium
Cl
Generally, transition metal complexes in MLCT excited
states are less reactive than in other excited states although
various complexes have been shown to be exceptions from
this rule [1,2]. Properties of MLCT excited states have been
examined for almost any type of p-acceptor ligand.
Recently, metal to NO+ MLCT states have attracted much
attention since the photolysis of such nitrosyl complexes
frequently leads to the release of NO [3,4] which plays an
important role in medicine. Aryldiazonium cations are iso-
electronic with NO+ and are also oxidants of considerable
strength [5,6]. Accordingly, MLCT transitions to aryl-N2þ
ligands and the photoreactivity originating from these
MLCT states should be quite interesting. So it is rather
surprising that this subject has apparently been completely
neglected. We explored this possibility and selected
the complex trichloro(phenylazo)bis(triphenylphosphine)-
ruthenium(II) [7] (RuCl3(N2Ph)(PPh3)2) for the present
study.
Ph
N
Ph3P
N
Ru
Cl
PPh3
Cl
This compound can be easily prepared and contains
Ru(II) as a well-known CT donor.
The electronic spectrum of RuCl3(N2Ph)(PPh3)2 in
CH3CN (Fig. 1) shows absorptions at kmax = 520 (sh,
e = 120 Mꢀ1 cmꢀ1), 420 (sh, 600), 310 (12,000) and 230
(sh, 18,300) nm. These solutions are light sensitive. Upon
irradiation with visible light a photolysis takes place as
indicated by the concomitant spectral changes (Fig. 2).
The final spectrum shows a new maximum at 395 nm and
a shoulder near 460 nm. This indicates the formation of
RuCl3(CH3CN)(PPh3)2 with
kmax = 395 nm (e = 3300)
and 464 nm (sh, 1300). An authentical sample of this com-
pound was prepared by the reaction of RuCl3 with a stoi-
chiometric amount of PPh3 in CH3CN [8]. The photolysis
of RuCl3(N2Ph)(PPh3)2 is also accompanied by evolution
*
Corresponding author. Tel.: +49 941 943 4716; fax: +49 941 943 4488.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
1387-7003/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.10.002

1186
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 1185–1186
at 310 nm. The complex disappears with / = 0.007 at
irr = 520 nm.
k
The compound RuCl3(N2Ph)(PPh3)2 is a typical octahe-
dral d6 complex. LF absorptions are expected to appear at
wavelength not longer then 400 nm [10]. It follows that the
longest-wavelength band at kmax = 520 nm can be only
assigned to
a
RuðIIÞ ! Ph ꢀ Nþ2 MLCT transition.
Ru(II) ! NO+ transitions occur at comparable energies
[11]. MLCT excitation of RuCl3(N2Ph)(PPh3)2 in CH3CN
proceeds according to the simple scheme:
RuIICl3ðN2PhÞðPPh3Þ2 !RuIIICl3ðCH3CNÞðPPh3Þ2 þPhN2
ð1Þ
PhN2 !N2 þPh
2Ph!biphenyl
ð2Þ
ð3Þ
Fig. 1. Electronic absorption spectrum of 2.79 · 10ꢀ5 M RuCl3(N2Ph)-
(PPh3)2 in CH3CN under argon at room temperature, 1-cm cell.
The photolysis is certainly facilitated by the lability of the
phenyl diazonium radical as the primary photoproduct
which undergoes an irreversible decay.
In summary, we have shown that the aryldiazonium cat-
ion is a strong CT acceptor which is photooxidized to phe-
nyl radicals upon MLCT excitation. Various applications
including initiation of radical polymerization are
conceivable.
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Fig. 2. Spectral changes during the photolysis of 1.56 · 10ꢀ5 M
RuCl3(N2Ph)(PPh3)2 in CH3CN at room temperature after 0 min (a), 1,
2 and 6 min (b) irradiation times with kirr > 420 nm (Osram HBO 200 W/2
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of nitrogen gas bubbles. While RuCl3(N2Ph)(PPh3)2 is not
luminescent the photolyzed solution displays a blue emis-
sion at kmax = 305 nm with a shoulder at 315 nm which
indicates the formation of biphenyl [9]. The photolysis is
monitored by following the decrease of the optical density
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(and references cited therein).