Photoreduction of carbonate in a copper(I) complex

Article abstract of DOI:10.1016/j.inoche.2009.10.020

The photolysis of (prophos)Cu^I(CO₃)Cu^I(prophos) with prophos = 1,3-bis(diphenylphosphino)-propane in homogeneous solution leads to the reduction of carbonate according to a simple overall stoichiometry (CO₂ + prophos → CO + prophos monoxide). It is suggested that this reaction is initiated by a (Cu^I → CO₃^{2 -}) MLCT state.

Full text of DOI:10.1016/j.inoche.2009.10.020

Inorganic Chemistry Communications 13 (2010) 137–138
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Photoreduction of carbonate in a copper(I) complex
*
Horst Kunkely, Arnd Vogler
Institut für Anorganische Chemie, Universität Regensburg, D-93040 Regensburg, Germany
a r t i c l e i n f o
a b s t r a c t
The photolysis of (prophos)CuI(CO3)CuI(prophos) with prophos = 1,3-bis(diphenylphosphino)-propane in
homogeneous solution leads to the reduction of carbonate according to a simple overall stoichiometry
Article history:
Received 28 September 2009
Accepted 26 October 2009
Available online 30 October 2009
(CO2 + prophos ? CO + prophos monoxide). It is suggested that this reaction is initiated by a ðCuI !CO2ꢀÞ
3
MLCT state.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Coordination chemistry
Copper(I)
Photochemistry
Carbonate reduction
The photoreduction of CO2 to CO by metal complexes has been
studied in some detail [1–3]. Frequently, complexes have been
examined which contain CO2 as ligand. In general, such complexes
are not available by simple procedures. In this context, it is quite
surprising that the photoreactivity of carbonate complexes has
been investigated only to a rather limited extent although the pho-
toreduction of carbonate ligands has been anticipated [4].
the reduction of carbonate to CO. Our expectation that complex I
should photochemically split off CO was based on various observa-
tions. The thermal release of CO from CO2 catalyzed by Cu(I) at
higher temperatures was already reported in 1976 [7,8]. Quite re-
cently, a related reaction has been discovered [9]. Moreover,
complex I shows certain analogies to (P/3)2CuI(NO3) [10]. As CT
acceptor it contains NOꢀ3 which is isoelectronic to CO23ꢀ. In this case
the irradiation leads to the population of a reactive MLCT state
which initiates the oxidation of Cu(I) und the reduction of nitrate.
The triphenylphosphine ligand does not only serve to stabilize
Cu(I), but provides also an IL state for light absorption. For our pur-
Carbonate complexes are known since the beginning of coordi-
nation chemistry. Generally, they are rather stable and easily
accessible. The reduction of CO23ꢀ to CO should be facilitated in
mononuclear or binuclear carbonate complexes with metal centers
which provide two excess electrons. The requested photoexcitation
corresponds to an MLCT transition which terminates at the carbon-
ate ligand as acceptor. However, MLCT absorptions of this type are
unknown. They are expected to occur at very short wavelength
pose the complex ðP/3Þ CuIð
l
-CO23ꢀÞCuIðP/3Þ should be a suitable
2
2
candidate. It was synthesized and characterized already in 1995
[11]. For further stabilization we replaced P/3 by the bidentate li-
gand prophos.
2ꢀ
since the
p
acceptor orbital of CO3 is certainly located at rather
The synthesis of I was achieved by the reaction of [CuI(CH3CN)4]
PF6 with Na2CO3 and prophos: [CuI(CH3CN)4]PF6 (745 mg, 2 mmol),
prophos (824 mg, 2 mmol) and Na2CO3 (210 mg, 2 mmol) were
suspended in an argon-saturated mixture of methanol (20 ml)
and acetonitrile (20 ml) and stirred in the dark for 6 days. The
resulting suspension was filtered. The slightly yellow filtrate was
added to a mixture of ether (50 ml) and pentane (150 ml). An
off-white, oily solid precipitated. It was collected by filtration,
washed with ether/pentane and dried. The desired salt I was ob-
tained as a white powder (170 mg, 18%). Anal. Calcd. for I: Calc.
(M = 1012.1) C 65.27, H 5.18, O 4.74, Cu 12.56. Found: C 65.14, H
5.26, O 4.68, and Cu 12.45.
*
high energies [5].
As a suitable candidate we selected the complex (prophos)CuI
(CO3)CuI(prophos) (I) with prophos = 1,3-bis(diphenylphosphino)
propane for the present study.
PPh2
CuI(µ-CO3
Ph2P
-
2
)
CuI
(I)
PPh2
Ph2P
Copper(I) is a MLCT donor and as a d10 metal it has not available
interfering LF states [6]. The oxidation of both metal centers to
Cu(II) leads to the release of two electrons which are required for
Complex I is colourless, well soluble in organic solvents and
does not decompose in humid air. The absorption spectrum of I
(kmax = 279 nm;
= 16,800) in CH3CN (Fig. 1) remains unchanged for days. How-
ever, I is light sensitive. The irradiation is accompanied by spectral
e
= 15 400 Mꢀ1 cmꢀ1 und kmax = 268 nm;
e
* Corresponding author. Tel.: +49 941 943 4716.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.10.020

138
H. Kunkely, A. Vogler / Inorganic Chemistry Communications 13 (2010) 137–138
Fig. 1. Spectral changes during the photolysis of 2.68 ꢁ 10ꢀ5
M
(pro-
phos)CuI(CO3)CuI(prophos) in acetonitrile at room temperature under argon at 0
(a), 15 and 40 min (b) irradiation times with kirr = 313 nm (Osram HBO 200 W/2
lamp), 1-cm cell.
Fig. 2. Potential energy diagram for the ground state and the lowest energy excited
states of (prophos)CuI(CO3)CuI(prophos).
variations including an isosbestic point at 231 nm (Fig. 1). During
irradiation of I is associated with an IL excitation. Partially, the
deactivation leads to an IL luminescence (kmax = 440 nm). The IL
emissions of comparable complexes [12] such as (P/3)2CuI(NO3)
at kmax = 450 nm [10] or [CuI(prophos)2]+ at kmax = 430 nm appear
in the same spectral region. Another deactivation path terminates
at a reactive ðCuI ! CO32ꢀÞ MLCT state which with participation of a
suitable vibration leads to the dissociation of CO from the carbon-
ate bridge (Fig. 2).
the photolysis
a continuous decrease of the absorption at
k > 231 nm takes place. The photoproduct shows a characteristic
spectrum (Fig. 1) with a maximum at 263 nm and further features
at kmax = 256, 271, 284 and 296 nm. Complex I displays a weak
luminescence at kmax = 440 nm (Fig. 2) which disappears during
the photolysis and is replaced by a much more intense lumines-
cence at kmax = 304 nm (Fig. 2). Further maxima appear at 282
(sh), 290 and 318 nm (sh).
According to this description the release of CO should be accom-
panied by the generation of a CuO2Cu moiety in the primary pho-
tochemical step. This fragment may contain CuIðO22ꢀÞCuI or its
redox isomer CuIIðO2ꢀÞ2CuII. Calculations have shown that they
are close in energy although such complexes are unknown
[13,14]. Since the CuO2Cu fragment should be quite oxidizing it
is not unexpected that it undergoes an oxygen transfer to a phos-
phine in agreement with our observation. It is well known that a
variety of oxygen transfer agents are able to convert phosphines
to phosphine oxides.
In summary, for the first time we describe the intramolecular
photoreduction of a carbonate ligand to CO. In addition, a phos-
phine ligand is transformed to a phosphine oxide as final oxidation
product. It is suggested that this photolysis is initiated by MLCT
excitation.
The absorption and emission spectrum of the photolyzed solu-
tion can be unambiguously attributed to 1,3-bis(diphenylphos-
phino)propane monoxide (prophos oxide, II) as shown by
comparison with the spectra of an authentic sample of II (ABCR,
Karlsruhe). From the difference of the extinction of II at 263 nm
(e = 4710) und 271 nm (e = 3768) its concentration can be deter-
mined reliably to some extent also in the presence of other absorb-
ing species provided they do not exhibit rather narrow absorptions
in this wavelength region. From the spectrum of the starting and
photolyzed solution equal molecular amounts of I and II are ob-
tained. The phosphine oxide II is formed with a quantum yield of
/ = 0.01 at kirr = 313 nm. Even at higher concentrations of I charac-
teristic LF bands of Cu(II) cannot be detected in the spectrum of the
photolyzed solution.
The photolysis of I is accompanied by the formation of a gas
which already in the beginning becomes visible as small bubbles.
This gas is not CO2. When it is introduced into an aqueous solution
of Na2CO3 which contains phenolphthalein as indicator the red
solution is not bleached by the formation of NaHCO3. The absorp-
tion of the red solution (kmax = 552 nm) remains unchanged. A gas
test specific for CO (gas detection pump accuro from Draeger
Safety) unambiguously shows that the gas formed during the pho-
tolysis of I is CO. This test can be also used for the quantitative
determination of CO but is less reliable at small amounts of gas.
In our case a molecular ratio of I to CO of 1:0.85 was obtained. In
addition, the gas volume was measured. For this purpose the pho-
tolysis was performed in a 3 ml cell which was connected to a gas
burette at constant pressure. The photolysis yielded a I to gas ratio
of 1:0.93 ( 5%). From these observations we conclude that the pho-
tolysis can be described by the simple stoichiometric equation:
Acknowledgement
Support of this research by the Deutsche Forschungsgemeins-
chaft (Grant No. VO 211/18-1) is gratefully acknowledged.
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CO2 þ prophos ! CO þ prophos monoxide
ð1Þ
On the basis of our observations and further considerations we
suggest that the photolysis proceeds by the following molecular
mechanism. In analogy to other Cu(I) phenylphosphine complexes
[12] including (P/3)2CuI(NO3) [10] the long-wavelength absorption
of I is attributed to an IL (phosphine) transition. Accordingly, the