Photochemically-induced ligand exchange reactions of ethoxy-oxo-molybdenum(V) tetraphenylporphyrin in chlorinated solvents

Article abstract of DOI:10.1016/j.poly.2006.06.027

Photochemically-induced ligand exchange reactions of ethoxy-oxo-molybdenum(V) 5,10,15,20-tetraphenylporphyrin, Mo(V)O(TPP)OEt, under irradiation at the Soret band region, were investigated. The reactions were performed in chlorinated solvents and followed with ultraviolet-visible (UV-vis) spectroscopy, whereas the products were analyzed with Fourier transformed infrared (FTIR) spectroscopy, electron spin resonance (ESR) spectroscopy and gas chromatography (GC). The chloro-oxo(TPP)Mo(V) complex was obtained as the final product, where the chlorine came from the solvent. Nevertheless, these reactions were not photocatalytic, due to the photochemical inertness of the formed chloro-oxo complex, and an excess of ethanol could not initiate a new photocatalytic cycle unless water was added as well, resulting in the formation of a biphasic system. On the other hand, addition of ethanol, water and NaOH to the reaction medium led to the formation of the corresponding oxo Mo(IV) complex. The studied photoassisted reactions of oxo Mo(V) porphyrins appear attractive for possible applications in the detection of chlorinated pollutants in sensing devices, but also for the dechlorination of pollutants upon suitable optimization of processing conditions.

Full text of DOI:10.1016/j.poly.2006.06.027

Polyhedron 25 (2006) 3427–3434
www.elsevier.com/locate/poly
Photochemically-induced ligand exchange reactions of
ethoxy-oxo-molybdenum(V) tetraphenylporphyrin
in chlorinated solvents
a,
a
b
Antonios M. Douvas *, Panagiotis Argitis , Andrea Maldotti ,
c,
*
Athanassios G. Coutsolelos
a
Institute of Microelectronics, National Center of Scientific Research Demokritos, 15310 Aghia Paraskevi, Athens, Greece
b
Department of Chemistry, Laboratory of Photochemistry, University of Ferrara, Ferrara, Italy
Department of Chemistry, Laboratory of Bioinorganic Coordination Chemistry, University of Crete, 71409 Iraklion, Crete, Greece
c
Received 24 March 2006; accepted 16 June 2006
Available online 6 July 2006
Abstract
Photochemically-induced ligand exchange reactions of ethoxy-oxo-molybdenum(V) 5,10,15,20-tetraphenylporphyrin, Mo(V)O(TPP)-
OEt, under irradiation at the Soret band region, were investigated. The reactions were performed in chlorinated solvents and followed
with ultraviolet–visible (UV–vis) spectroscopy, whereas the products were analyzed with Fourier transformed infrared (FTIR) spectros-
copy, electron spin resonance (ESR) spectroscopy and gas chromatography (GC). The chloro-oxo(TPP)Mo(V) complex was obtained as
the final product, where the chlorine came from the solvent. Nevertheless, these reactions were not photocatalytic, due to the photochem-
ical inertness of the formed chloro-oxo complex, and an excess of ethanol could not initiate a new photocatalytic cycle unless water was
added as well, resulting in the formation of a biphasic system. On the other hand, addition of ethanol, water and NaOH to the reaction
medium led to the formation of the corresponding oxo Mo(IV) complex. The studied photoassisted reactions of oxo Mo(V) porphyrins
appear attractive for possible applications in the detection of chlorinated pollutants in sensing devices, but also for the dechlorination of
pollutants upon suitable optimization of processing conditions.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Photoassisted ligand exchange reactions; Ethoxy-oxo(TPP)Mo(V) porphyrin; Detection of chlorinated pollutants; Sensing; Dechlorination;
Biphasic systems
1. Introduction
catalysis [3] but recently have expanded in areas such as
artificial photosynthesis [4,5], molecular electronics [6]
and chemical sensors [7–10]. Solid-state studies have been
also conducted, where supramolecular systems [4,5,11,12]
have been prepared and many film preparation methods
[13–16] have been proposed.
In the case of analytical applications of porphyrins, their
sensing function is usually based on either mass variation
or variation in their electronic spectra caused by weak
Van der Waals interactions, hydrogen bond interactions
or coordination of the central ion of the porphyrin with
the analyte [7a]. In this context, the coordination of
their axial ligands has been found to be very sensitive to
The capability of porphyrins to mediate electron and
energy transfer in a variety of systems, their electrocatalytic
and photocatalytic properties, and their significant role in
biological processes have attracted great attention and
have motivated research efforts towards a broad range of
applications [1,2]. The possible technological applications
of these compounds in the past were referred mainly in
*
Corresponding authors.
E-mail addresses: adouvas@imel.demokritos.gr (A.M. Douvas),
coutsole@chemistry.uoc.gr (A.G. Coutsolelos).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.06.027

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A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
environmental changes, which could be easily followed, for
instance by optical spectroscopy [7b].
Elmer FT series 1760 spectrophotometer in KBr pellets.
ESR spectra were obtained with a X-band Buker 220 SE
spectrometer. GC analyses were conducted with a DANI
8521 gas chromatograph equipped with a flame ionization
detector, using columns packed with Carbowax, 20 M, 5%
on Chromosorb W-AW.
Solutions of the ethoxy-oxo complex with concentration
2.4 · 10ꢀ4 M in various solvents were initially prepared.
More concentrated solutions of ꢁ10ꢀ3 M of this complex
were prepared only for detection reasons and it is explicitly
stated when used. The solutions of the ethoxy-oxo complex
were irradiated with a 500 W high-pressure Hg lamp (usu-
ally operated at 300 W) and the desired wavelength range
(450–455 nm) has been selected with an ORIEL
monochromator.
For the measurement of the quantum yield, e.g. of the
photochemical conversion of the ethoxy-oxo complex to
the chloro-oxo complex during the exposure at ꢁ450 nm,
the number of moles of photons absorbed was measured
with radiometry at this wavelength and the number of
moles of the ethoxy-oxo complex reacted was calculated
from the corresponding UV spectra: initially (absorbance
Ao at 450 nm) and after the irradiation (absorbance A at
450 nm), where Ao and A differ less than 10%. Repeated
experiments indicated that this method for the determina-
tion of the quantum yield had an error of up to 20%.
Oxomolybdenum(V) complexes were among the por-
phyrins studied in the past for both catalytic and analytical
purposes [1–3,17–27]. One point of interest in these com-
pounds is that they present ligand exchange behaviour sim-
ilar to that encountered in the biochemical function of iron
porphyrins. The main difference between the iron and
molybdenum porphyrins is that in the molybdenum case
the oxo ligand is placed in one of the two axial positions
and that ligand exchange reactions are based on the ease
of removal of the ligand placed at the trans position to this
oxo ligand [1,28–31]. These reactions are usually redox
ones, accompanied by oxidation or reduction of the metal,
and sometimes can be photoassisted. In the last case the
oxomolybdenum(V) porphyrins function as photoreducing
agents [32,33].
In this context, we are interested in investigating the
photoassisted ligand exchange reactions of oxomolybde-
num(V) porphyrins, mainly in view of possible sensing
applications, but also for the dechlorination of chlorinated
pollutants. Herein, we study the photochemical behaviour
of oxomolybdenum(V) porphyrins in different chemical
environments and under different conditions. More specif-
ically, we report on the photochemically-induced ligand
exchange reactions of the ethoxy-oxo(TPP)Mo(V) complex
mainly in chlorinated solvents and secondarily in non-
chlorinated solvents.
3. Results and discussion
2. Experimental
3.1. Photochemically-induced ligand exchange reactions
2.1. Materials
The first experimental evidence of photochemically-
induced ligand exchange reactions of ethoxy-oxo(TPP)-
Mo(V) porphyrin has been obtained during its elution
with dichloromethane in a chromatographic column. The
green solution of this porphyrin was unstable under labo-
ratory light and it was converted to a brown species, which
was identified with UV–visible spectroscopy as the chloro-
oxo derivative. More specifically, it was noticed that the
characteristic 452 nm peak of the ethoxy-oxo complex
was significantly reduced under laboratory light and the
characteristic 497 nm peak of the chloro-oxo complex
was formed, along with all the other peaks of that complex
mentioned in the literature [1,28] (see Fig. 1 and Table 1).
This photochemical reaction was further investigated
under controlled conditions. It was found that photoexcita-
tion of a dichloromethane solution of the ethoxy-oxo com-
plex (concentration 2.4 · 10ꢀ4 M) in the Soret band region
(450–455 nm) induced a ligand exchange reaction: the eth-
oxy ligand was replaced by the chloride one. UV–visible
spectroscopy indicated that the ethoxy-oxo complex was
completely (100%) converted to the chloro-oxo complex
(after 340 s of illumination under the experimental condi-
tions used), as shown in Fig. 1. The quantum yield of this
photochemical reaction at 450 nm was ꢁ38 · 10ꢀ4 (Table
1). The reaction progress shown in this figure is similar to
the one obtained through chloride anion addition to a
The starting chemicals were reagent grade and were used
without further purification, except as noted. The basic
type I alumina column was activated at 150 ꢁC for a mini-
mum of 24 h. H2(TPP) and Mo(CO)6 were the starting
materials used for the synthesis of the dimer l-oxo(TPP)-
Mo(V) according to a slightly modified (dichloromethane
was used instead of chloroform as eluant) Srivastava
method [34,35]. The synthesis was performed under an
argon stream atmosphere using Schlenk-tube techniques.
The dimer was identified with FTIR and UV–visible spec-
troscopy and the spectra obtained were compared with the
corresponding ones in the literature [28] (Fig. S1). Elution
of the reaction mixture from the basic activated alumina
column with dichloromethane, containing a small quantity
of ethanol for stabilization, resulted in the formation of the
ethoxy-oxo(TPP)Mo(V) complex, as was identified by UV–
visible spectroscopy.
2.2. Instrumentation and physicochemical measurements
Absorption spectra were collected on a Perkin–Elmer
Lambda 6 grating spectrophotometer and e (Mꢀ1 cmꢀ1
)
measurements were performed in CHCl3 solutions of
0.05 · 10ꢀ3 M. FTIR spectra were recorded on a Perkin–

A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
3429
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
452
0 sec
20
30
40
60
90
339
130
418
190
340
497
581
620
674
300
400
500
600
700
wavelength / nm
Fig. 1. UV–vis spectra evolution during Soret band irradiation of the ethoxy-oxo complex in dichloromethane. The ethoxy-oxo complex is completely
converted to the chloro-oxo complex. (The exposure time is written on each spectrum.)
Table 1
(a) UV–vis absorption data and (b) quantum yields of photochemical reactions of molybdenum porphyrinsa
(a)
Complex
Solvent
Wavelength (nm) (e · 10ꢀ3
M
ꢀ1 cmꢀ1
)
Ref.
Ethoxy-oxo Mo(V)
Chloro-oxo Mo(V)
Hydroxy-oxo Mo(V)
l-Oxo Mo(V)
dichloromethane
dichloromethane
dichloromethane
benzene
453(182), 540(3.5), 581(15.9), 620(10.9)
343(60), 417(3.3), 498(43), 628(8.6), 674(9.3)
464(90), 593(10.8), 635(8.4)
410(36), 444(59), 583(3.5), 617(4.2), 667(3.3)
429(295), 480(2.43), 511(3.79), 553(17.2), 590(3.4), 640(1.2)
[1,28]
[1,28]
[1,28]
[1,28]
[1,23]
Oxo Mo(IV)
dichloromethane
(b)
Initial complex
Solvent
Additive-concentration (%v/v)
CH2Cl2 (2.5%)
Photochemically formed complex
Quantum yield (U · 104)
Ethoxy-oxo Mo(V)
Ethoxy-oxo Mo(V)
Ethoxy-oxo Mo(V)
Ethoxy-oxo Mo(V)
dichloromethane
acetone
tetrachloroethylene
acetone
chloro-oxo Mo(V)
oxo Mo(IV)
chloro-oxo Mo(V)
chloro-oxo Mo(V)
38
13
8
3
2.5 0.5
2.5 0.5
a
The concentration of the initial ethoxy-oxo complex was 2.4 · 10ꢀ4 M.
methoxy-oxo(TPP)Mo(V) solution, as has been reported
before [23].
tion of the oxo-complex [1,23,28]. The quantum yield of
this photochemical reaction at 450 nm was ꢁ13 · 10ꢀ4
(Table 1).
The observed photoassisted ligand exchange process
may be explained on the basis of reactions (1)–(4). Accord-
ing to the literature [32,33], photolysis of the ethoxy-
oxo(TPP)Mo(V) porphyrin is expected to induce primarily
the reduction of the metalloporphyrin and the formation of
ethoxy radicals according to the following equation:
Evidence for the formation of ethoxy radicals was
obtained by irradiating ethoxy-oxo porphyrin solutions
containing phenyl-tert-butylnitrone (PBN) as a spin trap
[36a] in the cavity of an ESR spectrometer. This experiment
was carried out in the presence of an excess of ethanol
(2.5% v/v). The obtained spectrum reported in Fig. 3 con-
sists of a triplet of doublets with the following values of the
hyperfine splitting constants: aN = 14.2 and aH = 2.5 G.
This spectrum is consistent with the formation of a para-
magnetic adduct between PBN and the photogenerated
ethoxy radical [36b].
MoVOðTPPÞOEt þ hm ! MoIVOðTPPÞ þ CH3CH2OÅ ð1Þ
In agreement with this statement, irradiation of an acetone
solution of the ethoxy-oxo complex (concentration
2.4 · 10ꢀ4 M) in the Soret band provided evidence that
some amounts of the oxo(TPP)Mo(IV) complex are accu-
mulated. In fact, Fig. 2 shows a decrease of the ethoxy-
oxo complex absorption at 450 nm and the growth of a
new band at 420 nm, which can be ascribed to the forma-
Ethoxy radicals are powerful reducing agents [37] and
they are expected to react with dichloromethane resulting
in the ejection of Clꢀ, in accordance with Eq. (2) [38–41]:

3430
A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
0.5
0.4
0.3
0.2
0.1
0.0
450
0 sec
30
90
300
720
1320
420
300
400
500
600
700
wavelength / nm
Fig. 2. UV–vis spectra progression during Soret band irradiation of the ethoxy-oxo complex in acetone. The ethoxy-oxo complex is converted to the oxo
Mo(IV) complex. (The exposure times are shown.)
Fig. 3. ESR spectrum of the ethoxy-oxo complex (ꢁ10ꢀ3 M) in dichloromethane containing ethanol (2.5% v/v) and PBN that indicates the formation of a
paramagnetic adduct between PBN and the photogenerated ethoxy radical.
CH3CH2OÅ þ CH2Cl2!ÅCH2Cl þ Clꢀ þ Hþ þ CH3CHO
Clꢀ þ MoVOðTPPÞOEt þ HþꢀMoVOðTPPÞCl þ EtOH
ð2Þ
ð3Þ
The formation of acetaldehyde through Eq. (2) was
demonstrated by gas chromatographic analysis of irradi-
ated dichloromethane solutions of the ethoxy-oxo complex
(2.4 · 10ꢀ4 M) containing ethanol and water (2.5% v/v
each). In fact, we were able to detect with GC analysis
the acetal derivative of acetaldehyde, formed through its
reaction with EtOH under acidic conditions. Chain propa-
gation reactions that lead to further dechlorination of the
ÅCH2Cl radical are also possible, as has been reported with
other dechlorination photocatalysts [42]. The formation of
chloride ions through Eq. 2 was verified indirectly by the
formation of the chloro-oxo complex during reaction (3).
Thus, the dechlorination reaction (2) is likely to be fol-
lowed by the equilibrium reaction (3), where Clꢀ ions
replace the ethoxy ligand of the porphyrin:
Reaction (3) has already been reported [23,43] but we also
verified it: addition of HCl to the solution of the ethoxy-
oxo complex resulted in ligand exchange and the formation
of the chloro-oxo complex with the characteristic optical
spectrum. We also noticed that the photochemically
formed chloro-oxo complex was stable in the absence of
light and it did not reconvert to the initial ethoxy-oxo com-
plex, as observed from its optical spectrum.
The reduced complex formed by Eq. (1) is also able to
react with dichloromethane, leading to the dechlorination
of this solvent and to the formation of the chloro-
oxo(TPP)Mo(V) complex according to the following
equation:
MoIVOðTPPÞ þ CH2Cl2 ! MoVOðTPPÞCl þ ÅCH2Cl ð4Þ

A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
3431
A similar behaviour has been previously reported for
iron meso-tetraarylporphyrins and other iron porphyrins
dissolved in CCl4 or CHCl3 [38–40,44].
467 nm) in comparison with the spectra obtained before
in dichloromethane. On the other hand, the disappearance
of the characteristic 450 nm peak of the ethoxy-oxo com-
plex and the appearance of the characteristic 492 nm peak
of the chloro-oxo complex shows an almost identical
behaviour with the one observed in dichloromethane.
Additionally, the photochemical reaction in tetrachloro-
ethylene is significantly slower than in dichloromethane,
since the quantum yield of the ligand exchange reaction
in tetrachloroethylene (ꢁ2.5 · 10ꢀ4) is almost one order
of magnitude smaller than the one in dichloromethane
(Table 1). Similar photochemical behaviour of the ethoxy-
oxo complex was also observed in experiments with
other chlorinated solvents, such as 1,2-dichloroethane and
1,2,4-trichlorobenzene.
The photoinduced ligand exchange process was also
investigated in acetone containing a low percentage of a
chlorinated solvent, to be compared with results obtained
in pure acetone. Thus, an acetone solution of the ethoxy-
oxo complex (2.4 · 10ꢀ4 M) containing dichloromethane
(2.5% v/v) was irradiated at the Soret band. The ethoxy-
oxo complex was converted to the chloro-oxo complex,
but with lower efficiency than in pure dichloromethane.
Indeed, the quantum yield was ꢁ2.5 · 10ꢀ4, which is
almost one order of magnitude smaller than the one in
dichloromethane only (Table 1).
In an alternative route, it is possible that the photore-
duced complex is oxidized by oxygen forming the superox-
ide ion, O2ꢀ, which, in turn, has been reported to lead to
dechlorination reactions [25,45–48]. This dechlorination
pathway is expected to occur with lower efficiency, because
the opposite redox reaction, the reduction of oxo(TPP)-
molybdenum(V) porphyrins by the superoxide ion, has
been reported to take place in similar systems [20].
As it has been already reported [1,33] for analogous
compounds, the chloro-oxo complex did not undergo
photochemically-induced ligand exchange reactions, a
behaviour attributed to the high polarity of the axial coor-
dination bond. It is generally known that the more ionic
the axial coordination bond of an oxomolybdenum(V)
complex the less readily the complex is photoreduced,
because this bond is cleaved homolitically upon photoirra-
diation [1,33].
3.2. Effects of the reaction medium on the photoproduct
formation
3.2.1. Homogeneous systems
The photoassisted ligand exchange reaction of the eth-
oxy-oxo metalloporphyrin was also investigated in tetra-
chloroethylene, in the presence of oxygen. Thus, a
tetrachloroethylene solution of the ethoxy-oxo complex
(concentration 2.4 · 10ꢀ4 M) was irradiated at the Soret
band. The ethoxy-oxo complex was completely (100%)
converted to the chloro-oxo complex after ꢁ72 min expo-
sure via the same photochemical ligand exchange reaction,
as shown in Fig. 4. In this figure, two isosbestic points
appear again, but these points are red shifted (at 384 and
Spectral evidence indicates that the described photoas-
sisted formation of the chloro-oxo metalloporphyrin
occurs also when irradiation is carried out in the presence
of a large excess of ethanol (molar ratio EtOH/complex
ꢁ220:1) in dichloromethane solution. This is a clear indica-
tion that equilibrium (3) remains shifted to the right despite
the high concentration of the ethanol added. The high sta-
bility of the chloro-oxo complex in the presence of ethanol
was confirmed by the finding that addition of ethanol
2.5
0 sec
20
450
350
2.0
1.5
1.0
0.5
0.0
410
1310
2510
341
492
3110
4310
579
621
669
300
400
500
600
700
wavelength / nm
Fig. 4. UV–vis spectra evolution during Soret band exposure of the ethoxy-oxo complex in tetrachloroethylene. The ethoxy-oxo complex is completely
converted to the chloro-oxo complex. (The exposure times are given; compare with data of Fig. 2.)

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A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
(0.5% v/v) to a dichloromethane solution of the photo-
chemically obtained chloro-oxo complex (2.4 · 10ꢀ4 M)
did not induce any spectral changes. An undesired conse-
quence of these results is that the restoration of the initial
ethoxy-oxo complex for initiating a new photocatalytic
cycle is not possible by simple addition of ethanol in excess.
tems enables us to regenerate the starting ethoxy-oxo por-
phyrin, so inducing the photoassisted ligand exchange
process in a cyclic manner.
Addition of ethanol and water (2.5% v/v each) to a
dichloromethane solution of the photogenerated chloro-
oxo complex (2.4 · 10ꢀ4 M) resulted in its quantitative
conversion to the ethoxy-oxo derivative, as was observed
with UV–visible spectroscopy (Fig. 5a). Under these condi-
tions, water removes high amounts of Clꢀ ions from the
organic phase, where the metalloporphyrin complex is dis-
solved, favouring in this way the detachment of Clꢀ from
its axial position and the coordination of ethanol. Subse-
quently, Soret band irradiation for 170 min of the last solu-
tion resulted in the complete formation of the chloro-oxo
3.2.2. Biphasic systems
The results discussed so far indicate that the photochem-
ical conversion of the ethoxy-oxo complex to its chloro-oxo
derivative puts an end to the desired photoassisted ligand
exchange process, due to the high stability of the chloro
complex, also in the presence of an excess of ethanol. In
the following, we demonstrate that the use of biphasic sys-
2.5
a
454
2.0
1.5
489
1.0
0.5
0.0
300
400
500
600
700
wavelength / nm
2.5
2.0
1.5
1.0
0.5
0.0
454
b
0 sec
488
30
180
240
420
720
337
415
1020
581
620
672
300
400
500
600
700
wavelength / nm
Fig. 5. UV–vis spectra progression during: (a) addition of ethanol and water (2.5% v/v each) to the photochemically converted chloro-oxo complex
(2.4 · 10ꢀ4 M in dichloromethane), which resulted in the formation of the initial ethoxy-oxo complex; (b) Soret band irradiation (for 170 min) of the
ethoxy-oxo complex formed in case (a), which resulted in the formation of the chloro-oxo complex.

A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
3433
5
4
3
2
1
0
428
450
10 min
30, 70
130
250
380
321
553 580 620
300
400
500
600
700
wavelength / nm
Fig. 6. UV–vis spectra evolution during Soret band exposure of a dichloromethane solution of the ethoxy-oxo complex (2.4 · 10ꢀ4 M) containing EtOH,
H2O and aqueous NaOH solution 1 M (2.5% v/v each). The ethoxy-oxo complex is completely converted to the oxo(TPP)Mo(IV) complex.
complex through the same photochemical ligand exchange
route (Fig. 5b). In this case, the whole heterogeneous solu-
tion was irradiated, but in fact the organic phase was
exposed and the UV–visible spectrum of this phase was
obtained, as the small quantity of the added water occupied
the bottom of the cell. In Fig. 5b, the complete disappear-
ance of the characteristic 454 nm peak of the ethoxy-oxo
complex and the appearance of the characteristic 488 nm
peak of the chloro-oxo complex are presented.
Consequently, the formation of the biphasic system
(dichloromethane/water with a major concentration of eth-
anol) of the ethoxy-oxo complex favours the photocatalytic
inducement of the previous ligand exchange route. The
photocatalytic cycles are terminated as soon as the whole
quantity of ethanol in the organic phase is photooxidised.
New catalytic cycles can be achieved by the biphasic system
with addition of a further quantity of ethanol. Thus, the
studied photoassisted ligand exchange process could be
used for the dechlorination of pollutants upon addition
of ethanol and water forming a biphasic system.
In order to increase the photocatalytic cycles of the
photoassisted ligand exchange process, ethanol, water
and NaOH were added to a dichloromethane solution of
the ethoxy-oxo porphyrin. More specifically, ethanol,
water and aqueous NaOH solution 1 M (2.5% v/v each)
were added to a dichloromethane solution of the ethoxy-
oxo complex (2.4 · 10ꢀ4 M). Subsequently, Soret band
irradiation of the solution for 380 min led to the complete
formation of the oxoMo(IV) complex, as was observed by
UV–vis spectroscopy (Fig. 6). The formation of the
hydroxy-oxo(TPP)Mo(V) complex, which could be
expected in this case, was not observed (i.e. the major
464 nm peak of the hydroxy-oxo complex along with the
other peaks of that complex were not observed, see Fig. 6
and Table 1). On the other hand, both the characteristic
peaks at 428 and 553 nm of the oxoMo(IV) complex were
obtained. Furthermore, detection of the acetal derivative in
this irradiated biphasic system with GC analysis was not
feasible, due to the predominant basic conditions. There-
fore, in this last, quite complicated biphasic system, the
photoassisted ligand exchange route was changed, as no
chloro-oxo complex was observed. A possible explanation
is that NaOH removes more effectively the Clꢀ ions from
the organic phase than water does. Nevertheless, further
investigation on this system is necessary in order to clarify
the picture.
4. Conclusions
In this contribution, the photoassisted ligand exchange
reactions of ethoxy-oxo(TPP)Mo(V) porphyrin were stud-
ied. In this first investigation specific attention has been
given to the formation of the photochemically inert
chloro-oxo complex, which results in a net overall photoas-
sisted ligand exchange reaction. This reaction is easily fol-
lowed due to the characteristic UV–visible spectra
evolution in different chlorinated solvents. Regeneration
of the starting ethoxy-oxo complex and, thus, photocata-
lytic inducement of the ligand exchange route was not pos-
sible by simple addition of ethanol in excess, but only with
the addition of water and ethanol in the reaction medium,
where a biphasic system is formed. Further addition of
NaOH changed the photoassisted ligand exchange route,
and led to the formation of the oxoMo(IV) complex,
instead of the chloro-oxo complex. The mechanistic details
and the possible practical value of the described chemistry
in bonded chlorine detection and dechlorination of pollu-
tants appear worthy of further investigation.

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A.M. Douvas et al. / Polyhedron 25 (2006) 3427–3434
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