Photooxidation of Rh(PPh3)2(CS)Cl induced by metal-to-ligand charge transfer excitation

Article abstract of DOI:10.1016/S0022-328X(98)01079-1

The longest-wavelength absorption of Rh^I(PPh₃)₂(CS)Cl at λ_max=452 nm is assigned to a metal-to-ligand charge transfer (MLCT) transition which terminates at the π* orbital of the thiocarbonyl ligand. MLCT excitation of the complex in CH₂Cl₂ initiates a photooxidation which finally leads to Rh^III(PPh₃)₂(CS)Cl₃ as a stable photoproduct with φ=0.003 at λ_irr=436 nm.

Full text of DOI:10.1016/S0022-328X(98)01079-1

Journal of Organometallic Chemistry 577 (1999) 358–360
Preliminary Communication
Photooxidation of Rh(PPh3)2(CS)Cl induced by metal-to-ligand charge
transfer excitationꢀ
Horst Kunkely, Arnd Vogler *
Institut fu¨r Anorganische Chemie, Uni6ersita¨t Regensburg, D-93040 Regensburg, Germany
Received 5 October 1998
Abstract
The longest-wavelength absorption of RhI(PPh3)2(CS)Cl at umax=452 nm is assigned to a metal-to-ligand charge transfer
(MLCT) transition which terminates at the p* orbital of the thiocarbonyl ligand. MLCT excitation of the complex in CH2Cl2
initiates a photooxidation which finally leads to RhIII(PPh3)2(CS)Cl3 as a stable photoproduct with ƒ=0.003 at uirr=436 nm.
© 1999 Elsevier Science S.A. All rights reserved.
Keywords: Photochemistry; Charge transfer; Rhodium; Thiocarbonyl
1. Introduction
Thiocarbonyl complexes [6] share some basic features
with carbonyl complexes, but may be quite different
with regard to their spectral and photochemical proper-
ties. Since p-bonding in CS is much weaker than in CO
the p* orbitals of CS are located at much lower energies
[7,8]. It follows that MLCT transitions to the CS ligand
occur also at comparably low energies [9–11]. Indeed,
the longest-wavelength absorption of W(CO)5CS is ap-
parently of the MLCT (W“p* CS) type [9,10]. Unfor-
tunately, this MLCT state seems to be quite unreactive.
W(CO)5CS undergoes only a photosubstitution with
low efficiency. This photolysis may originate from a LF
state which is situated above the MLCT state.
However, it is now well known that a variety of
metal complexes are characterized by reactive MLCT
states [12]. Thiocarbonyl complexes are not expected to
be an exception. We explored this possibility and se-
lected the complex Rh(PPh3)2(CS)Cl [13,14] for the
present study. This choice was guided by the following
considerations. This compound is easily prepared and
quite stable. Generally, Rh(I) complexes with p accept-
ing ligands show longest-wavelength MLCT absorp-
tions in their electronic spectra [15]. Moreover, MLCT
Metal carbonyls comprise the largest and most im-
portant class of light-sensitive organometallic com-
pounds [1–3]. Generally, carbonyl complexes undergo
photochemical ligand substitutions which are induced
by ligand-field (LF) excitation with long-wavelength
irradiation. On the other hand, metal-to-ligand charge
transfer (MLCT) transitions terminating at the p* or-
bitals of CO are formally associated with the oxidation
of the metal and reduction of the ligand. This redox
process which is related to the water gas shift reaction
[4] could be exploited for various applications. Unfortu-
nately, ML (p* CO) CT transitions occur only at very
high energies and are thus not easily accessible. Accord-
ingly, very little is known on the reactivity of such
MLCT states [5].
ꢀ Dedicated to Professor Helmut Werner, on the occasion of his
65th birthday.
* Corresponding author. Tel.: +49-941-9434485; fax: +49-941-
9434488.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01079-1

H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 577 (1999) 358–360
359
of this oxidative addition is Rh(PPh3)2(CS)Cl3. The
photolysis is monitored by measuring the decrease of
the optical density at 253 nm taking into account the
residual absorption of Rh(PPh3)2(CS)Cl3 at this wave-
length (m=8300). Rh(PPh3)2(CS)Cl disappears with
ƒ=0.003 at uirr=436 nm.
In CH3CN solutions Rh(PPh3)2(CS)Cl is hardly light
sensitive if oxygen is excluded. However, in the pres-
ence of O2 a photolysis takes place. The concomitant
spectral changes are similar but not identical to those
which are observed during the photolysis in CH2Cl2.
Fig. 1. Electronic absorption spectra of 8.06×10−5
M
RhI(PPh3)2(CO)Cl (a, ---) and 5.65×10−5 M RhI(PPh3)2(CS)Cl
3. Discussion
(b, —) in CH2Cl2 under argon at r.t, 1 cm cell.
In analogy to various other Rh(I) complexes [15–17]
with p-accepting ligands the longest-wavelength absorp-
tion of Rh(PPh3)2(CO)Cl at umax=364 nm has been
assigned to a MLCT transition which terminates at the
excitation may yield Rh(III) as a stable photooxidation
product. Finally, it should be useful to compare the
behavior of Rh(PPh3)2(CS)Cl with Rh(PPh3)2(CO)Cl.
The spectral [16,17] and photochemical [18–20] proper-
ties of the latter complex are well known.
phosphine
ligand
[16,17].
The
complex
Rh(PPh3)2(CS)Cl displays its lowest-energy band at dis-
tinctly longer wavelength (umax=452 nm) (Fig. 1). This
absorption is then logically assigned to the MLCT
transition from Rh(I) to the p* orbital of the thiocar-
bonyl ligand. This assignment is supported by related
observations on M(CO)5CS (M=Cr [11] and W [9,10])
and ReI(CO)4(S2COEt) [21].
Rh(PPh3)2(CO)Cl is well known to undergo a pho-
toejection of CO [18–20]. It seems likely that this
photodecarbonylation originates from a LF state in
agreement with the general behavior of metal carbonyls
[1–3]. Although LF bands are not apparent in the
spectrum of Rh(PPh3)2(CO)Cl they may be present at
relatively long wavelength but obscured by the more
intense MLCT absorption at 364 nm. Reactive LF
states can thus be populated from the MLCT state
which is initially reached by light absorption.
2. Results
The electronic spectrum of Rh(PPh3)2(CS)Cl in
CH2Cl2 (Fig. 1) shows absorptions at umax=452 (m=
1300 M−1 cm−1), 338 (4500), 290 (11300) and 253 nm
(sh, 25600). The spectrum is only slightly dependent on
the solvent. In CH3CN some band maxima are shifted
by
a
few nanometers. The absorptions of
Rh(PPh3)2(CO)Cl [16,17] in CH2Cl2 (Fig. 1) appear at
umax=364 (2800), 290 (12200) and 256 nm (18600).
Both complexes are not emissive at room temperature
(r.t.) or 77 K.
Solutions of Rh(PPh3)2(CS)Cl in CH2Cl2 are light
sensitive. The photolysis is accompanied by spectral
changes (Fig. 2) which can be duplicated by the thermal
reaction of Rh(PPh3)2(CS)Cl with Cl2 [13]. The product
The ML (p* CS) CT state of Rh(PPh3)2(CS)Cl is
located at rather low energies and apparently well
separated from higher LF states. The photoreactivity of
the complex is obviously initiated by MLCT excitation.
In CH2Cl2 a photoredox reaction takes place. It can be
described by the following equation:
RhI(PPh3)2(CS)Cl+2CH2Cl2
“RhIII(PPh3)2(CS)Cl3+2 · CHCl2
Product formation may proceed in two steps. A pri-
mary electron transfer from the MLCT excited complex
to solvent leads to the generation of Rh(II) which is
subsequently oxidized by a second CH2Cl2 molecule in
a thermal process. One or two-electron photooxidations
of metal complexes by the solvent, in particular chlori-
nated alkanes, are frequently induced by MLCT excita-
tion [12,22]. As an alternative, in non-oxidizing solvents
such as CH3CN molecular oxygen may serve
as an oxidant. In this case the photoproduct of
Fig. 2. Spectral changes during the photolysis of 7.11×10−5
M
RhI(PPh3)2(CS)Cl in CH2Cl2 under argon at r.t. after 0 (a), 15 (b), 30
(c), 60 (d) and 120 (e) min irradiation time with uirr=366 nm (Xe/Hg
977 B-1 lamp), 1 cm cell.

360
H. Kunkely, A. Vogler / Journal of Organometallic Chemistry 577 (1999) 358–360
[3] D.M. Roundhill, Photochemistry and Photophysics of Metal
Complexes, Ch. 6, Plenum, New York, 1994.
[4] For recent Refs., see: R. Ziessel, J. Am. Chem. Soc. 115 (1993)
118.
Rh(PPh3)2(CS)Cl was not identified but the spectral
similarity with RhIII(PPh3)2(CS)Cl3 suggests that a re-
lated Rh(III) complex such as [Rh(PPh3)2(CS)
(CH3CN)2Cl]2+ is generated.
In conclusion, it is shown that the lowest-energy ML
(p* CS) CT state of Rh(PPh3)2(CS)Cl initiates a pho-
toredox reaction which leads to the formation of
RhIII(PPh3)2(CS)Cl3 and oxidized solvent (CH2Cl2). It is
also feasible that other suitable substrates react with the
MLCT excited complex by intercepting the reduced CS
ligand.
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Acknowledgements
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Support of this research by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemis-
chen Industrie is gratefully acknowledged.
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