Photoreactivity of the triphenylphosphine adduct of methyl(trioxo)rhenium(VII) induced by ligand-to-metal charge transfer excitation

Article abstract of DOI:10.1016/S0022-328X(00)00274-6

Triphenylphosphine forms an adduct with MeReO₃. The yellow colour of this compound is attributed to a (PPh₃ → Re^VII) ligand-to-metal charge transfer (LMCT) absorption at λ_max = 380 nm. LMCT excitation (λ_irr = 405 nm) leads to the reduction of Re(VII) to Re(V). This reaction has been previously observed as a thermal process (Herrmann et al., Organometallics 13 (1994) 4531).

Full text of DOI:10.1016/S0022-328X(00)00274-6

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Journal of Organometallic Chemistry 606 (2000) 207–209
Communication
Photoreactivity of the triphenylphosphine adduct of
methyl(trioxo)rhenium(VII) induced by ligand-to-metal charge
transfer excitation
Horst Kunkely, Arnd Vogler *
Institut fu¨r Anorganische Chemie, Uni6ersita¨t Regensburg, D-93040 Regensburg, Germany
Received 13 January 2000; received in revised form 28 February 2000; accepted 27 April 2000
Abstract
Triphenylphosphine forms an adduct with MeReO3. The yellow colour of this compound is attributed to a (PPh3“ReVII
)
ligand-to-metal charge transfer (LMCT) absorption at umax=380 nm. LMCT excitation (uirr=405 nm) leads to the reduction of
Re(VII) to Re(V). This reaction has been previously observed as a thermal process (Herrmann et al., Organometallics 13 (1994)
4531). © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Photochemistry; Charge transfer; Rhenium; Phosphines
stabilise transition metals in low oxidation states. With
regard to electronic spectroscopy and photochemistry
1. Introduction
phosphines are mainly spectator ligands although their
Organic phosphines (or phosphanes) PR3 play an
direct involvement in intraligand (IL) [1] and metal-to-
important role as ligands in coordination chemistry in
ligand charge transfer (MLCT) [2,3] excited states has
general and organometallic chemistry in particular. Ow-
been recently observed in a few cases. In this context it
ing to their p-acceptor properties they are able to
is quite surprising that despite their well-known reduc-
ing ability very little is known on phosphines as donor
ligands for CT transitions. Jensen and Jørgensen esti-
mated the optical electronegativity of PEt3 (ꢀ2.6) on
the basis of ligand-to-metal charge transfer (LMCT)
assignments of absorption bands in the electronic spec-
tra of Co(PEt3)2X2 with X=Cl and Br [4]. However,
these complexes may not be well suited for an unam-
biguous detection of LMCT absorptions since Co(II) is
hardly oxidising. It follows that phosphine complexes
of oxidising metals should be much better candidates to
identify LMCT transitions and to explore their pho-
Fig. 1. Electronic absorption spectra of 1.25×10−2 M MeReO3 and
toreactivity. In order to avoid any interference by lig-
1.25×10−2 M PPh3 in CH2Cl2 as (a) separate solutions (—) and (b)
and field (LF) transitions oxidising d0 complexes would
as a mixture (- - -) under argon at 5°C, 1 cm tandem cell.
be most suitable to investigate CT spectra of complexes
with phosphines as donor ligands. As an appropriate
* Corresponding author. Tel.: +49-941-9434485; fax: +49-941-
9434488.
example we selected the complex MeReVIIO3·PPh3 for
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
the present study. This choice was based on the follow-
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00274-6

208
H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 606 (2000) 207–209
3. Discussion
The yellow colour which instantly develops when
solutions of MeReO3 and PPh3 are mixed is attributed
to the complex MeReO3(PPh3). Such a facile adduct
formation has been observed with a variety of other
ligands [5,6]. The new absorption maximum at 380 nm
is then logically assigned to a LMCT transition from
the reducing phosphine ligand to the oxidising d0 metal
centre. This assignment is supported by additional evi-
dence. When PPh3 is replaced by P(p-Clꢀphenyl)3 the
LMCT band is shifted to shorter wavelength and ap-
pears only as a shoulder near ꢀ315 nm. The substitu-
tion of the phenyl rings by chlorine certainly reduces
the donor strength of the phosphine and increases thus
the energy of a LMCT transition. The photoreactivity
of MeReO3(PPh3) can now be attributed to a PPh3“
ReVII LMCT excited state. We suggest that the primary
photochemical step consists of the reduction of
MeReO3 by the PPh3 ligand:
Fig. 2. Spectral changes during the thermal reaction of 5.3×10−4
MeReO3(PPh3) in CH2Cl2 under argon at r.t. after (a) 0, (b) 10 and
(c) 40 min.
M
ing considerations. MeReO3 is a pseudooctahedral d0
complex that easily expands its coordination sphere by
accepting further ligands [5,6]. The electronic spectra
and the photochemistry of MeReO3 [7–9] and several
adducts MeReO3·L [9–12] have been examined in some
detail. Although MeReO3·PPh3 is apparently not a
stable compound observations on its thermal decompo-
sition [13] were expected to facilitate the current
investigation.
MeReVIIO3(PPh3)ꢀꢀꢀLꢀMꢀꢀCꢀTꢀꢀꢁ MeReVO2+OꢂPPh3
u
=405 nm
irr
Although the optical LMCT transition is a one-electron
transfer from PPh3 to Re(VII) the LMCT excited state
should easily relax to redox products which are the
result of an oxygen atom transfer from MeReO3 to the
phosphine ligand. Owing to the reducing properties of
PPh3 this reaction occurs also as a thermal process [13].
The primary product MeReVO2 which is generated
thermally or photochemically is apparently coordina-
tively unsaturated, but can be stabilised in the presence
of an excess of MeReO3 and PPh3 [13]:
2. Results
When colourless solutions of MeReO3 and PPh3 in
CH2Cl2, ether or n-hexane are mixed a yellow colour
develops immediately. This colour is caused by an
absorption at umax=380 nm (Fig. 1). At 10−2
M
equimolar concentrations of both components the ap-
parent extinction coefficient at 380 nm is m=85 M−1
cm−1. Since an increase of the PPh3 concentration does
not lead to an increase of the extinction it is assumed
that the 1:1 adduct MeReO3·PPh3 is responsible for the
380 nm absorption. If the temperature is lowered from
room temperature (r.t.) to 0°C the adduct is stable for
ꢀ30 min. At r.t. the adduct undergoes a thermal
reaction at much shorter periods. This reaction is ac-
companied by a colour change from yellow to red and
brown [13]. The concomitant spectral variations are
shown in Fig. 2. The product of this reaction is
MeReV(PPh3)2O2·MeReVIIO3 which had been charac-
terised previously [13]. This reaction does not only
proceed thermally but also as a rather efficient photore-
action. The photolysis of the adduct MeReO3·PPh3 in
ether at 0°C with uirr=405 nm generates the same
spectral changes as those of Fig. 2. At later stages of
the photolysis the pattern of the spectral variations
changed indicating secondary processes. Nevertheless, a
rough estimate yielded an extinction coefficient of m=
200 at 510 nm for the primary photoproduct. The
quantum yield of product formation was ƒ=0.09 at
uirr=405 nm.
MeReVO2+2PPh3+MeReVIIO3
“MeReV(PPh3)2O2·MeReVIIO3
The binuclear ReV/ReVII mixed-valence compound is
formed as final product.
In summary, the complex MeReO3(PPh3) represents
a first example which clearly demonstrates the donor
ability of phosphines for optical CT. Moreover, the
photoreactivity of this complex reflects the charge dis-
tribution in the PPh3“ReVII LMCT excited state.
Acknowledgements
Support of this research by the Fonds der Chemis-
chen Industrie is gratefully acknowledged.
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