inorganic chemistry

Numerous molecular metal complexes have been examined as
photocatalysts in homogenous solution. The reduction of water
to hydrogen and the oxidation to oxygen can be studied as inde-
pendent processes. The photoreduction of water or protons to
hydrogen is a two-electron transfer that may be mediated by var-
ious mononuclear and binuclear (or polynuclear) metal
complexes. They can undergo an oxidation to a stable oxidation
state that is higher by two units. A prominent type of a metal
complex that generates H 2 is a hydride complex which undergoes
reductive elimination upon irradiation, for example(7,8),

IrIIICl HðÞ
 2 ðÞPPh 33!

hn

IrICl PPhðÞ 33 þH 2 ð 8 Þ

Generally, hydride is an electron donor or reductant. Accordingly,
(H
to IrIII) LMCT excitation could initiate the observed photoly-
sis. However, how can this photoredox reaction be related to water
or proton reduction? While various hydride complexes undergo
such a reductive elimination of H 2 their synthesis is generally
based on complicated procedures and cannot be formed by a sim-
ple reaction with water or protons in aqueous solution( 8 ).
Even more difficult, the photooxidation of water to oxygen is
associated with the transfer of four electrons. In natural photo-
synthesis, this is accomplished by photosystem II, which
contains a tetranuclear manganese cluster as active oxidation
center. In artificial systems, various complexes have been shown
to generate oxygen upon irradiation( 9 ). This applies also to per-
manganate but the mechanism of oxygen release seems to be a
sequence of quite complicated steps ( 10 ). An interesting example
is a binuclear complex containing the FeIIIOFeIII moiety, for
example (edta¼ethylendiaminetetraacetate) ( 11 ):

2 edtaðÞFeIII
O
FeIIIðÞedta

hi 4 


!

hn

4 edtaðÞFeII

hi 2 


þO 2 ð 9 Þ

It is suggested that (O^2
!FeIII) LMCT and/or (FeIII!FeIII)
MMCT excitation initially leads to the disproportionation of Fe(III).

ðÞedtaFeIII
O
FeIIIðÞedta

hi 4 


! ðÞedtaFeIV¼¼O

hi 2 

þ ðÞedtaFeII

hi 2 
 ð^10 Þ

The oxoferryl fragment FeIVO is not stable. The subsequent reac-
tion of two such complexes yields FeIIand oxygen:

2 edtaðÞFeIV¼¼O

hi 2 


!2 edtaðÞFeII

hi 2 


þO 2 ð 11 Þ

348 ARND VOGLER AND HORST KUNKELY