Laser flash photolysis experiments carried out for a wide range of
initial parameters, provided new evidence that intramolecular elec-
tron transfer from a ligand to the Fe(III) center is the main photo-
chemical process in the photochemistry of [FeIII(C 2 O 4 ) 3 ]^3
( 222 ).
As primary transient species, the radical complex
[(C 2 O 4 ) 2 FeII(C 2 O 4 )]^3
was detected; the yield of organic radicals in
the primary photoprocess was found to be negligible. A kinetic
schemeforthephotolysisof[FeIII(C 2 O 4 ) 3 ]^3
wasproposed,including
formationof[(C 2 O 4 ) 2 FeII(C 2 O 4 )]^3
,itsreversibledissociationtothe
oxalateionandsecondaryradicalcomplex[(C 2 O 4 )FeII(C 2 O 4 )]
,and
the decay of both radical complexes. Quite a similar behavior was
reported for the heteroleptic Fe(II) oxalate complexes withN,N-
ethylenebis(benzoylacetoneiminato) Schiff base derivatives ( 223 ).
Recently, the photochemical redox cycling of iron coupled to the
oxidationofthemalonateligand(CHCOO) 22
hasbeeninvestigated
under conditions that are representative of atmospheric waters
( 224 ). Malonate exhibited significantly different characteristics
from oxalate and other dicarboxylates. Spin-trapping electron spin
resonance experiments proved the formation of both the
CH 2 COOHand OHradicals at lower total malonate concentra-
tion, but only CH 2 COOHat higher concentrations of malonate,
providing strong evidence for competition between malonate and
OH
ligands, and subsequent different photoreaction pathways.
The study is of relevance to advance our understanding of iron
cycling in acidified carbon-rich atmospheric waters.
Moreover, recent studies on Fe(III) complexes with eth-
ylenediamine-N,N-disuccinate ((CH 2 NHCHCH 2 (COO) 2 ) 24
, edds),
which is a structural isomer of edta, proved that the photolysis
of Fe-polycarboxylates can be diverse(2,95,225). Irradiated sol-
utions of [Fe(edds)]
showed generation of OHradicals with a
quantum yield dependent on pH and independent of the complex
concentration. Thus, the proposed reaction scheme was as follows:

FeIIIðÞedds

hi

!

hnðÞLMCT

FeIIþedds ð 35 Þ

FeII
!

edds;O 2

FeIIIðÞedds

hi


ð 36 Þ

edds^3 
þOH
!edds^4 
þOH ð 37 Þ

This study gave evidence for the potential role of [FeIII(edds)]
as
a photoactive species in natural waters.
The ligand radicals generated by mono- and polycarboxylate
iron(III) complexes can also be the driving force for photocatalytic
oxidative degradation of external pollutants directly or upon their

METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 321