Inorganic Chemistry Communications
Short communication
Photochemistry of Os(bipy) (CO ). Photoreduction of carbonate
2
3
to formaldehyde
Arnd Vogler
Institute of Inorganic Chemistry, University Regensburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
2
The complex Os(II)(bipy) (CO ) undergoes a photoredox reaction in acetonitrile/H O yielding
2
3
Received 23 April 2016
Received in revised form 8 June 2016
Accepted 11 June 2016
2+
2 2
[Os(VI)(bipy) (O) ]
and formaldehyde. The reactive excited state is suggested to be of the (Os(II) to π* carbon-
ate) MLCT type.
©
2016 Elsevier B.V. All rights reserved.
Available online 14 June 2016
Keywords:
Photochemistry
Photoredox reaction
Carbonate
Carbon dioxide
Osmium
The photochemical reduction of carbon dioxide is of fundamental
importance in the field of biochemistry. Accordingly, many efforts
have been devoted to understand and to simulate this part of natural
photosynthesis. As a key step of an artificial photosynthesis it is consid-
ered to be an important contribution to the utilization of solar irradia-
tion as a regenerative energy source provided by chemistry. It has
been shown that transition metal complexes can facilitate this photore-
duction by sensitizing and catalyzing this process [1–4]. In order
to avoid an undesired interference, in particular by the intermediate for-
mation of radicals, the reduction should proceed by a transfer of an even
mercury lamp Osram HBO 100 W/2. Formaldehyde was identified by
the Merckoquant test stripes.
The complex Os(bipy) (CO ) is soluble in CH Cl and polar solvents
2 3 2 2
such as acetonitrile (AN). In our case most experiments were performed
in mixtures of acetonitrile and water (1:1 vol%). Upon irradiation of
Os(bipy)
which suggest that Os(bipy)
2 3
(CO ) in AN/water spectral changes are observed (Fig. 1),
2
+
2 3 2 2
(CO ) is converted to [Os(bipy) (O) ]
[6]. The spectrum of the dioxo complex is characterized by an absorp-
tion maximum near 400 nm, in addition to the much more intense
band at λmax = 300 nm [6].
number of electrons. For example, the reduction of CO
may yield CO. In a few cases this has been accomplished [3,4]. The pho-
toreduction of CO with 4 or 6 electrons should give formaldehyde or
methanol, respectively. We explored this possibility and selected the
complex Os(II)(bipy) (CO ) [5] (bipy = 2,2′-bipyridyl) for the present
2
by two electrons
The photolyzed solution shows a weak, but distinct luminescence
(Fig. 2) with λmax = 638 nm. Moreover, formaldehyde was identified
as a further photolysis product. A gas evolution which should take
place when the carbonate ligand would undergo a two-electron reduc-
tion to CO was not observed.
2
2
3
study. The oxidation of Os(II) complexes of this type tends to yield
Os(VI) as stable product [6,7]. Moreover, carbonate as a ligand is better
When the solution of Os(bipy)
2 3
(CO ) was treated with increasing
amounts of Ce(NO , the spectral variations (Fig. 3) were nearly
3 4
)
accessible than CO
2
. In both cases the 4-electron reduction should lead
identical to those of the photolysis (Fig. 1). In this case it is well
known that the oxidation by Ce(IV) leads to the formation of
to the generation of formaldehyde. Finally, there is an opportunity to
photoreduce Os(VI) back to Os(II) in order to provide a cyclic process,
which could be considered as a photocatalysis.
2
+
2 2
[Os(VI)(bipy) (O) ]
[6].
2 3
The absorption spectrum of Os(II)(bipy) (CO ) in the visible region
The complex Os(bipy)
procedure [5].
2
(CO
3
)
was prepared by
a
literature
displays bands which are of the (Os(II) to π* bipy) MLCT type [5].
MLCT excitation with λirr N 380 nm is essentially not effective, but
shorter-wavelength irradiation (with white light) leads to an efficient
photolysis which proceeds according to the Eq. (1).
Emission spectra were obtained on a Hitachi 850 spectrofluorimeter
equipped with a Hamamatsu 928 photomultiplier. The luminescence
spectra were recorded for monochromator and photomultiplier effi-
ciency variations. Solutions were irradiated in 1-cm quartz cells at r.t.
The photolyses were performed by irradiation with a high-pressure
Â
Ã
2þ
OsðIIÞðbipyÞ ðCO3Þ þ 2 H2O→ OsðVIÞðbipyÞ ðOÞ
þ H2CO
2
2
2
−
þ 2 OH
ð1Þ
1387-7003/© 2016 Elsevier B.V. All rights reserved.