Photolysis of CH3Re(O2)2O induced by ligand-to-metal charge transfer and by peroxide intraligand excitation

Article abstract of DOI:10.1016/j.inoche.2005.02.014

The electronic spectrum of the diperoxo complex MeRe^VII(O ₂)₂O shows a peroxide IL (intraligand) absorption at λ_max ～ 260 nm in addition to the well-known peroxide to Re(VII) LMCT band at λ_max = 358 nm. Upon IL excitation, the diperoxo group undergoes a dismutation. Accordingly, MeRe(O₂) ₂O photolyzes to MeRe^VIIO₃ and O₂. LMCT excitation at λ_irr = 405 nm is assumed to generate the radical pair MeReVI(O2)O+/O2- in the primary photochemical step. Back electron transfer and reaction with water leads finally to the monoperoxo complex MeRe^VII(O₂)O₂ and H₂O₂.

Full text of DOI:10.1016/j.inoche.2005.02.014

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Inorganic Chemistry Communications 8 (2005) 467–470
www.elsevier.com/locate/inoche
Photolysis of CH3Re(O2)2O induced by ligand-to-metal
charge transfer and by peroxide intraligand excitation
*
Horst Kunkely, Arnd Vogler
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, Universitaetsstrasse 31, D-93040 Regensburg, Germany
Received 16 January 2005; accepted 2 February 2005
Available online 24 March 2005
Abstract
The electronic spectrum of the diperoxo complex MeReVII(O2)2O shows a peroxide IL (intraligand) absorption at kmax ꢀ 260 nm
in addition to the well-known peroxide to Re(VII) LMCT band at kmax = 358 nm. Upon IL excitation, the diperoxo group under-
goes a dismutation. Accordingly, MeRe(O2)2O photolyzes to MeReVIIO3 and O2. LMCT excitation at kirr = 405 nm is assumed to
generate the radical pair MeReVIðO2ÞOþ=Oꢁ in the primary photochemical step. Back electron transfer and reaction with water
2
leads finally to the monoperoxo complex MeReVII(O2)O2 and H2O2.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Electronic spectra; Photochemistry; Rhenium; Peroxo complexes; Charge transfer
A variety of peroxo complexes has been shown to be
light sensitive [1–10]. The photosensitivity is apparently
associated with the peroxo ligand. The nature of the
reactive excited states should be related to the chemical
properties of peroxide. Since O22ꢁ is reducing as well as
oxidizing, it can serve as CT donor and acceptor, respec-
tively. In combination with an oxidizing metal, LMCT
(ligand-to-metal charge transfer) excited states are pres-
ent at low energies and apparently are responsible for
the light sensitivity. When coordinated to a reducing me-
tal, the photolysis may be initiated by MLCT (metal-to-
ligand charge transfer) excitation [5,6]. Finally, peroxide
itself has available p* ! r* transitions at relatively low
energies. We suspected that this p* ! r* IL (intrali-
gand) excitation of diperoxo complexes could induce
the disproportionation of peroxide.
The product formation would finally be the result of
an interligand CT process within the ðO22ꢁÞ moiety. The
2
photochemical release of O2 from complexes such as
MoVI(O2)2O(L) [7,8] and CH3ReVII(O2)2O [9,10] may
point to such a mechanism. However, the complicated
product distribution obscures such a reaction path.
Our present study provides some evidence that the per-
oxide disproportionation initiated by peroxide intrali-
gand excitation of a diperoxo complex indeed occurs.
In agreement with previous reports, we observe that
solutions of MeRe(O2)2O are light sensitive [9,10].
According to these studies and some general consider-
ations, we expected that MeRe(O2)2O, MeReO3 and
HReO4 are important potential products of the photol-
ysis of MeRe(O2)2O. Since the spectra of these com-
plexes are known [9–13], an analysis of the spectral
variations which accompany the photolysis may reveal
the appearance of these complexes as primary and sec-
ondary photoproducts. First of all, we observe that the
course of the photolysis of MeRe(O2)2O in water-satu-
rated ether depends on the wavelength of irradiation.
At kirr = 254 nm, the optical density above 300 nm is
decreasing proportional to the original extinction
ðO22ꢁÞMnþðO22ꢁÞ ! Mnþ þ 2O2ꢁ þ O2 or MnþO2 þ O2
ð1Þ
*
Corresponding author. Tel.: +49 941 9434485; fax: +49 941
9434488.
E-mail address: arnd.vogler@chemie.uniregensburg.de (A. Vogler).
1387-7003/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2005.02.014

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H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 467–470
(Fig. 1). It follows that only MeReO3 and HReO4 can
be the products since they do not absorb above 300
and 280 nm [13], respectively. In contrast, MeRe(O2)2O
strongly absorbs at longer wavelength [9,11,12]. During
the first stage of the photolysis of MeRe(O2)2O, the
original spectrum can be almost completely recovered
by treating the photolyzed solution with H2O2. Under
our conditions, HReO4 does not react with H2O2 to
generate an absorbing peroxo complex such as
H4Re2O13 [14], but MeReO3 is completely converted
to MeRe(O2)2O [11,12]. It follows that essentially only
MeReO3 is formed as primary photoproduct upon irra-
diation of MeRe(O2)2O with 254 nm light. However, at
later stages of the photolysis, the optical density
continues to decrease, but upon addition of H2O2 to
the photolyzed solution the original spectrum of
MeRe(O2)2O is only partially recovered. When the
photolysis of MeRe(O2)2O is driven to completion only
HReO4 is left. Accordingly, HReO4 is produced as
secondary photolysis product by the irradiation of
MeReO3. This photoreaction which takes place in the
presence of water is well known [13,15].
At kirr = 405 nm, the photolysis of MeRe(O2)2O in
water-saturated ether takes a different course. The con-
comitant spectral changes (Fig. 2) show a general
decrease of the optical density above 300 nm, but not
proportional to the starting extinction at different wave-
lengths. In particular, the shift of the absorption maxi-
mum from 358 to 310 nm (shoulder) is suspicious.
These spectral variations are compatible with the forma-
tion of MeRe(O2)2O2 (kmax = 310 nm) [11] and MeReO3.
The latter photoproduct does not undergo any second-
ary photolysis under these conditions because it does
not absorb light above 300 nm [13]. Upon addition of
H2O2 to the photolyzed solution, the starting complex
MeRe(O2)2O is completely regenerated as indicated by
the reversal of the spectral changes. This observation
agrees with the chemical behavior of MeRe(O2)2O2 and
Fig. 2. Spectral changes during the photolysis of 5.85 · 10ꢁ4 M
MeRe(O2)2O in ether at room temperature after 0 min (a), 5, 10 and
25 min (b) irradiation times with kirr = 405 nm (Osram HBO 200 W/2
lamp; Schott monochromatic interference filter MA 71 405), 1 cm cell.
MeReO3. Both complexes are known to form
MeRe(O2)2O upon addition of H2O2 [11,12]. At later
stages of the photolysis at kirr = 405 nm, the spectral pat-
tern becomes irregular indicating secondary thermal and/
or photochemical reactions which are probably associ-
ated with MeRe(O2)O2.
Generally, peroxo complexes of d0 metals such as
Ti(IV), V(V), Cr(VI) and Mo(VI) are characterized by
long-wavelength (O22ꢁ ! d0 metal) LMCT absorptions
which frequently determine the colors of these com-
pounds [16]. There is little doubt that this LMCT assign-
ment applies also to the longest-wavelength bands
of MeRe(O2)O2 (kmax = 310 nm) and MeRe(O2)2O
(kmax = 358 nm) [11]. These LMCT assignments are sup-
ported by recent calculations on peroxo complexes of
Ti(IV) [17], Mo(VI) [18–21] and Re(VII) [21,22] which
have shown that the HOMO is largely derived from
the occupied p-antibonding orbitals of peroxide, while
the LUMO contains a considerable d-orbital contribu-
tion from the d0 metal. However, in addition these cal-
culations have shown that an empty MO which is
essentially based on the r*-orbital of peroxide is located
only at slightly higher energies than the metal-based
LUMO. It follows that peroxo complexes of d0 metals
are expected to display peroxide IL p* ! r* transitions
at accessible energies above the lowest-energy LMCT
transition. We suggest that this O22ꢁ IL transition
belongs to the absorption of MeRe(O2)2O at
kmax ꢀ 260 nm (shoulder). Similar bands appear also
in the spectra of other peroxo complexes such as MoV
(O2)2O [7] at comparable energies. In distinction to
LMCT transitions, IL transitions of this type should
be only slightly affected by the metal center. Moreover,
the IL ðO22ꢁÞ assignment is in agreement with the photo-
chemical properties of MeRe(O2)2O.
Fig. 1. Spectral changes during the photolysis of 4.64 · 10ꢁ4 M
MeRe(O2)2O in ether at room temperature after 0 min (a), 10, 20
and 40 min (b) irradiation times with kirr = 254 nm (Osram HBO
200 W/2 lamp; Schott monochromatic interference filter UVIL 254),
1 cm cell.
Upon peroxide IL excitation (kmax = 254 nm), the
photolysis of MeRe(O2)2O in ether essentially proceeds
according to the simple equation

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H. Kunkely, A. Vogler / Inorganic Chemistry Communications 8 (2005) 467–470
469
MeReVIIðO2Þ O ! MeReVIIO3 þ O2
ð2Þ
excited state which could be populated from the initially
excited LMCT state by an activated process. However,
MeRe(O2)O2 as second photoproduct is assumed to be di-
rectly generated by LMCT excitation. In this context, it is
quite interesting that LMCT excitation of MoVI(O2)2O
leads also to the formation of a monoperoxo complex
[7]. We suggest that the photolysis of MeRe(O2)2O initi-
ated by (O22ꢁ to ReVII) LMCT excitation proceeds accord-
ing to the following equation:
2
This conclusion is also supported by previous obser-
vations. It has been reported that the quantum yield for
the release of O2 from MeRe(O2)2O increases towards
shorter wavelengths and reaches unity at kirr = 248 nm
[9]. It is of interest that this observation has also led to
the conclusion that the photolysis is initiated by a higher
excited state but its nature has not been discussed.
In a simplified picture, the photochemical changes of
MeRe(O2)2O induced by IL excitation are restricted to
MeReVIIðO22ꢁÞ O ! MeReVIðO22ꢁÞOþ ꢂ ꢂ ꢂ O2ꢁ
radical pair formation
2
the peroxide ligands: ðO22ꢁÞ ! 2O2ꢁ þ O2. While the
ð3Þ
2
primary optical transition certainly takes place in the
delocalized ðO22ꢁÞ moiety, the subsequent rearrange-
MeReVIðO22ꢁÞOþ . . . Oꢁ þ H2O
2
2
ment is associated with an electron transfer from one
! MeReVIIðO22ꢁÞO2 þ H2O2
peroxide ligand to the other. The ðO22ꢁÞ fragment can
2
back electron transfer and protonation of O22ꢁ ð4Þ
be viewed as ligand-based mixed-valence (MV) system
(Fig. 3) in analogy to certain metal-based MV com-
plexes [23,24]. After the initial excitation (indicated by
the vertical arrow), both peroxide ligands are still equiv-
alent, but then the electron density is shifted from one
peroxide to the other. Accordingly, the equivalence of
both peroxide ligands is lifted. This electron transfer is
accompanied by structural changes (e.g., left side of
Fig. 3) which finally lead to the dismutation of peroxide
(Scheme 1).
Generally, LMCT excitation generates a radical pair
[25]. This applies also to Re(VII) complexes including
MeReO3 [13,15,26]. In the case of MeRe(O2)2O, the rad-
ical pair should be composed of a Re(VI) complex and
superoxide. Frequently, radical pairs formed by LMCT
excitation undergo a back electron transfer. As a result,
a photosubstitution may take place [25]. In the present
case, this step is certainly facilitated by water which
protonates the released peroxide and provides the miss-
ing oxide ligand for the monoperoxo complex as final
photoproduct.
The qualitative potential energy diagram (Fig. 3) shows
that this reaction cannot only be achieved photochemi-
cally, but takes also place as an thermal process which
requires an activation energy (Ea = 14.7 kcal molꢁ1 [9]).
In conclusion, the diperoxo complex MeRe(O2)2O
undergoes a photosubstitution of one peroxide ligand
upon low-energy LMCT excitation (kirr = 405 nm).
Moreover, shorter-wavelength irradiation at 255 nm leads
to peroxide IL excitation which is followed by electron
Upon ðO22ꢁ ! ReVII
Þ
LMCT excitation (kirr =
405 nm), the photolysis of MeRe(O2)2O in watercontain-
ing ether takes a different course. Since one of the photo-
products is also MeReO3, it may originate from the IL
transfer within the ðO22ꢁÞ ligand fragment. Finally, a dis-
2
mutation takes place: ðO22ꢁÞ ! 2O2ꢁ þ O2. As a result,
2
the starting diperoxo complex is photolyzed to MeReO3.
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