inorganic chemistry

The primary photoproduct TpCuII(OH) 2 CuIITp is itself also
photoactive. The photolysis of TpCuII(OH) 2 CuIITp in CH 3 CN
at rather low concentrations (< 10
^4 M) was accompanied by
spectral changes that are roughly the reversal of those inFig. 4.
At higher concentrations of TpCuII(OH) 2 CuIITp, this photoly-
sis led to the precipitation of CuITp*, which is rather insoluble
in most solvents. It was identified by its absorption and emission
spectrum withlmax¼594 nm. As further products O 2 and H 2 O 2
were detected and determined. The ratio of H 2 O 2 to O 2 decreased
with increasing irradiation times. After extensive photolysis
essentially only oxygen was detected while in the beginning only
H 2 O 2 was apparently formed. A quantitative analysis roughly
established Eq. (29) for this photolysis after short irradiation
times.

TpCuIIðÞm-OH 2 CuIITp!2TpCuIþH 2 O 2 ð 29 Þ

The quantum yield for the disappearance of
TpCuII(OH) 2 CuIITp wasf¼ 3  10
^3 atlirr¼313 nm. The com-
plex was not light sensitive upon long wavelength irradiation
(l>450 nm) in the region of the Cu(II) LF bands.
It follows fromEqs. (28) and (29) that the photosplitting of
dialkyl carbonates should be catalytic. This is expressed by
Eq. (30) or (31), which includes the disproportionation of H 2 O 2.

CO ORðÞ 2 þ2H 2 O!COþH 2 O 2 þ2ROH ð 30 Þ

CO ORðÞ 2 þH 2 O!COþ

1

2

O 2 þ2ROH ð 31 Þ

Indeed, extended irradiation (l>290 nm) of CuITp in CO(OR) 2 /
H 2 O mixtures was associated with formation of CO exceeding the
amount calculated forEq. (28) with TON>5. The limitation of
photocatalysis in our system is apparently caused by an
accompanying irreversible photodecomposition of Cu(I) and/or
Cu(II) Tp complexes as indicated by the precipitation of a
slightly colored material after continued irradiation.
At this point a few comments about the mechanism of the pho-
tochemical splitting of carbonate are appropriate. We suggest
that in agreement with the excited state properties of Tp
complexes in general(46,47) the photoexcitation of CuITp leads
to the population of an IL excited state followed by electron
transfer to the carbonate. A radical anion is thus formed as an
intermediate which is subsequently reduced by a second CuITp
complex forming CO. Simultaneously, two CuIITp* fragments
are generated as oxidation products. Under these conditions

PHOTOCHEMICAL ACTIVATION AND SPLITTING OF H 2 O 359