Photocatalysis of chloroform decomposition by hexachloroosmate(IV)

Article abstract of DOI:10.1111/j.1751-1097.2009.00669.x

Hexachloroosmate(IV) effectively catalyzes the photodecomposition of chloroform in aerated solutions. The decomposition products are consistent with a mechanism in which excited state OsCl₆^2- reduces chloroform, rather than one involving photodissociation of chlorine atoms. Trace amounts of ethanol or water in the chloroform lead to photosubstitution to form OsCl₅(EtOH)^- or OsCl₅(H₂O) ^-, neither of which is photocatalytically active.

Full text of DOI:10.1111/j.1751-1097.2009.00669.x

Photochemistry and Photobiology, 2010, 86: 467–470
Research Note
Photocatalysis of Chloroform Decomposition by Hexachloroosmate(IV)
˜
Laura A. Pena and Patrick E. Hoggard*
Department of Chemistry, Santa Clara University, Santa Clara, CA
Received 20 July 2009, accepted 9 October 2009, DOI: 10.1111/j.1751-1097.2009.00669.x
roosmate(IV) is dominated by intense LMCT bands (19)
ABSTRACT
although lower energy d–d excited states are also present, and
internal conversion to this manifold could lead to different
primary photochemical events, or none at all. The chlorine
atoms formed by photodissociation can initiate radical chain
processes in which an organic substrate is degraded, poten-
tially to CO2 and HCl.
Hexachloroosmate(IV) effectively catalyzes the photodecompo-
sition of chloroform in aerated solutions. The decomposition
products are consistent with a mechanism in which excited state
2)
OsCl6 reduces chloroform, rather than one involving photo-
dissociation of chlorine atoms. Trace amounts of ethanol or
water in the chloroform lead to photosubstitution to form
OsCl5(EtOH)) or OsCl5(H2O)), neither of which is photocat-
alytically active.
MATERIALS AND METHODS
(Bu4N)2OsCl6 was prepared by dissolving K2OsCl6 (Sigma-Aldrich)
and Bu4NBr in water, mixing the two solutions, collecting the
precipitate by filtration, and air-drying. Chloroform (J.T. Baker
reagent grade) was prepared by shaking with an equal volume of
water five times to remove the ethanol stabilizer. It was then dried over
anhydrous calcium chloride.
Photolyses were carried out by pipetting 3.0 mL of a solution into a
fused silica cuvette and irradiating it with either a 350-W or a 100-W
mercury lamp (Oriel) with a Schott WG-320 cutoff filter, which may be
characterized approximately as passing wavelengths above 320 nm.
Some samples were deoxygenated by bubbling argon through the
cuvette for 10 min. UV-visible spectra were monitored with a Cary 50
spectrophotometer. GC-mass spectrometry was carried out with a
Shimadzu QP-5000 instrument with a Restek XTI-5 column. The oven
start temperature was 40ꢀC and a linear temperature gradient of
30ꢀmin)1 was applied to 250ꢀC. Injection was carried out with a 1:1
split ratio. Chlorine-containing species were identified from their mass
spectra.
HCl production was measured by periodically removing 50 lL
aliquots from the photolysate, and adding them to 3.0 mL of a
solution of tetraphenylporphyrin (H2TPP) in CHCl3, the spectrum of
which was then measured to determine the amount of H4TPP2+
formed, by using the extinction coefficients of the porphyrin species
(20,21). Protonation of H2TPP yields H4TPP2+ in preference to
H3TPP+, even at low concentrations of acid (22).
Peroxide concentrations were estimated by mixing 50 or 100 lL
aliquots of the photolysate with 3.00 mL of approximately 0.01 M
Bu4NI in CHCl3 and determining the resulting I3) concentration from
the extinction coefficient, 2.50 · 104, at 365 nm (23), correcting for the
absorption from OsCl62) and taking into account the experimental 1:1
ratio of hydroperoxide reacted to triiodide ion formed (24). Triiodide
ion is also formed by the reaction of I) with Cl2, although GC-MS
analysis showed the concentration of Cl2 to be minor in comparison to
the concentration of total oxidants.
INTRODUCTION
Chloroform and other trihalomethanes that find their way into
drinking water primarily through municipal disinfection (1,2)
have half-lives of months in aqueous environments even
exposed to the sun (3). Chloroform is one of the water
pollutants most commonly found to exceed regulatory guide-
lines (1,4,5). Materials that could heterogeneously catalyze the
photodecomposition of chloroform by means of sunlight
would have the potential to effect passive remediation;
however, it is not yet clear how such a catalyst might function.
We have examined several metal complexes that homoge-
neously catalyze chloroalkane decomposition in the neat
solvent. Some of these function through photodissociation of
a chlorine atom (6,7), which generally requires irradiation into
ligand-to-metal charge transfer (LMCT) bands, which facili-
tate metal–ligand bond homolysis (8–11). An alternative route
to photodegradation consists of the photoreduction of the
chloroalkane (12–16). In either case, radicals are created that
react with oxygen to form peroxy radicals and hydroperoxides,
leading to further decomposition through a chain process (17).
Yet another means to effect photodecomposition is for the
catalyst to undergo photoreduction by oxidizing a counterion
or another substrate to create radicals, following which the
catalyst is restored by a thermal process that may generate
additional radicals (18). To function in aqueous environments,
coordinated chlorides, at least some of them, should resist
hydrolysis. This eliminates chloro complexes of first-row
transition metals as candidates.
The initial concentrations of (Bu4N)2OsCl6 in chloroform solutions
were calculated from absorption spectra, using the value of 9.78
)1
(
0.01) · 103
M
cm)1 at 341 nm, determined from a Beer’s Law plot.
2)
In this study, we examined OsCl6 as a potential photo-
catalyst in chloroform. The near-UV spectrum of hexachlo-
RESULTS AND DISCUSSION
Hexachloroosmate dissolved in chloroform did catalyze its
photodecomposition. Figure 1 shows HCl produced during
*Corresponding author email: phoggard@scu.edu (Patrick E. Hoggard)
ꢁ 2009 The Authors. JournalCompilation. The AmericanSocietyofPhotobiology 0031-8655/10
467

468 Laura A. Pen˜a and Patrick E. Hoggard
GC-MS analysis of photolysates provides evidence that
photoreduction, Eqs. (9) and (10), constitute a more likely
mechanism for chloroform photodecomposition than photo-
dissociation (Eqs. [1] and [2]). From the generation of dichlo-
romethyl radicals by photoreduction, as in Eq. (9), one would
expect to find CH2Cl2 from hydrogen abstraction, as in Eq.
(10), along with C2H2Cl4, C2HCl5 and C2H6 from the termina-
tion of •CHCl2 and •CCl3 radicals, respectively. All three of
these products were found in significant amounts in photoly-
sates. Typically, the concentrations of CH2Cl2, C2H2Cl4 and
C2HCl5 were approximately 20%, 10% and 40% of the
concentration of C2Cl6. From the photodissociation mecha-
nism, C2Cl6 is the only chloroethane termination product
expected and experimentally this is what is observed in systems
in which chloroform is known to be degraded through the
photodissociation of chlorine from a metal complex (7).
Another prediction from the two proposed mechanisms
would be that in deoxygenated solutions some HCl would still
be produced through Eq. (2) if photodissociation occurred,
while from photoreduction either a much smaller amount of
HCl would be produced or none at all, depending on how the
)
OsCl6 intermediate is reduced (vide infra). In fact, no HCl at
all was detected in irradiated deoxygenated solutions.
Figure 1. Accumulation of HCl and peroxide (in equivalents relative
to Os) during the broadband (k > 320 nm) irradiation of a 2 · 10)4
M
2)
The OsCl6
spectrum remained essentially unchanged
solution of (Bu4N)2OsCl6 in chloroform that was exposed to air and
from which ethanol was removed.
under irradiation (k > 320 nm); thus, no significant amount
of OsCl6) accumulated and it must have been rapidly reduced.
Given the relative difficulty of oxidizing chloroform by
electron transfer, chloride ion (from the reduction of CHCl3,
Eq. [9]) is the most probable substrate. One possibility,
observed in some other photocatalytic systems (16,18), is for
chloride ion to be oxidized to chlorine radicals. This would
lead to some HCl formation through hydrogen abstraction by
the irradiation of a typical aerated sample. In the absence
of the osmium complex, no HCl was detected after 40 min of
irradiation. During the course of irradiation, the spectrum of
2)
the OsCl6
Chlorine atoms generate HCl through hydrogen abstraction;
(kmax = 341 and 375 nm) did not change.
thus the result is consistent with
mechanism:
a
photodissociation
chlorine atoms, which was not observed. This is consistent
) ⁄ 2)
with the experimental value for the OsCl6
half-wave
OsCl26ꢀꢀ! OsCl52ꢀ þ Clꢁ
ð1Þ
ð2Þ
hm
potential, 1.52 V (vs. NHE) in CH2Cl2 (29), whereas the
Cl ⁄ Cl) potential (in water) is 2.5 V (30). An alternative is a
two-electron oxidation to Cl2, which may require the partici-
pation of a coordinated chloride, i.e.
Cl ꢁ þCHCl3 ꢀ! HCl þ ꢁCCl3
In aerated solutions, however, HCl can also be generated in
a chain process through the formation of a hydroperoxide,
OsClꢀ6 þ Clꢀ ꢀ! OsCl52ꢀ þ Cl2
OsCl25ꢀ þ CHCl3 ꢀ! OsCl62ꢀ þ CHCl2
ð11Þ
ð12Þ
2)
which also constitutes a means of regenerating the OsCl6
starting material.
ꢁCCl3 þ O2 ꢀ! CCl3OOꢁ
CCl3OO ꢁ þCHCl3 ꢀ! CCl3OOH þ ꢁCCl3
ð3Þ
ð4Þ
In Eq. (12) the osmium(III) intermediate is presumed to
2)
reduce chloroform in order to be restored to OsCl6 to
complete the cycle. Experimentally, Cl2 was found in photol-
ysates by GC-MS. This offers some experimental support for
the suggested mechanism, whereby it must be noted that an
analogous cycle has been reported by Maverick et al. for
2ꢀ
CCl3OOH þ OsCl25ꢀ ꢀ! CCl3Oꢁ þOsCl5ðOHÞ
ð5Þ
2ꢀ
OsCl5ðOHÞ þHCl ꢀ! OsCl26ꢀ þ H2O
ð6Þ
ð7Þ
ð8Þ
2)
CCl3Oꢁ ꢀ! COCl2 þ Clꢁ
COCl2 þ H2O ꢀ! CO2 þ 2 HCl
hexachlororhenate(IV), in which ReCl6 is photooxidized to
ReCl6), which is then reduced by Cl), yielding Cl2. The
)
reaction was proposed to pass through a Cl2 intermediate:
Since hydroperoxides can be formed from the reaction of
any radical with oxygen, it cannot be concluded from this
experiment alone that HCl is formed through photochemical
Os–Cl homolytic bond cleavage. An alternative, based on
several observations of the photoreduction of chloroalkanes by
metal complexes (25–28) proceeds as follows:
ReClꢀ6 þ 2 Clꢀ ꢀ! ReCl26ꢀ þ Clꢀ2
2 Clꢀ2 ꢀ! Cl2 þ 2 Clꢀ
ð13Þ
ð14Þ
Replacing Re by Os, this presents a reasonable alternative
to Eqs. (11) and (12), and from either cycle it would be
predicted that no HCl would form. It is not entirely clear how
the three reactants in Eq. (13) could combine to yield the
proposed products, for which reason we have suggested that
OsCl26ꢀ þ CHCl3ꢀ! OsCl6ꢀ þ ꢁCHCl2 þ Clꢀ
ꢁCHCl2 þ CHCl3 ꢀ! CH2Cl2 þ ꢁCCl3
ð9Þ
hm
ð10Þ

Photochemistry and Photobiology, 2010, 86 469
5. U.S. Environmental Protection Agency. (2002) National primary
drinking water standards. EPA Document 816-F-01-007.
6. Doyle, K. J., H. Tran, M. Baldoni-Olivencia, M. Karabulut
and P. E. Hoggard (2008) Photocatalytic degradation of
dichloromethane by chlorocuprate(II) Ions. Inorg. Chem. 47,
7029–7034.
7. Hoggard, P. E., M. Gruber and A. Vogler (2003) The photolysis of
iron(III) chloride in chloroform. Inorg. Chim. Acta 346, 137–142.
8. Horvath, O. and A. Vogler (1993) Photoredox chemistry of
chloromercurate(II) complexes in acetonitrile. Inorg. Chem. 32,
5485–5489.
9. Oldenburg, K. and A. Vogler (1993) Electronic spectra and pho-
tochemistry of tin(II), lead(II), antimony(III), and bismuth(III)
bromide complexes in solution. Z. Naturforsch., B: Chem. Sci. 48,
1519–1523.
10. Oldenburg, K. and A. Vogler (1996) Photoredox chemistry
of bismuth trichloride in benzene. J. Organomet. Chem. 515,
245–248.
11. Vogler, A. and A. Paukner (1989) Photoredox chemistry of chloro
complexes of antimony(III) and (V). Inorg. Chim. Acta 163, 207–
211.
12. Pena, L. A., A. J. Seidl, L. R. Cohen and P. E. Hoggard (2009)
Ferrocene ⁄ ferrocenium ion as a catalyst for the photodecompo-
sition of chloroform. Transition Met. Chem. (Dordrecht, Neth.) 34
135–141.
13. Traverso, O., R. Rossi and V. Carassiti (1974) Improved photo-
chemical method for obtaining ferricenium cation. Synth. React.
Inorg. Met.-Org. Chem. 4, 309–315.
one of the two chlorides to be oxidized might be taken from
the coordination sphere.
The lack of a significant photodissociation pathway, given
that absorption takes place into LMCT charge transfer bands,
is not difficult to reconcile with the electronic structure in
OsCl62). Whereas FeCl4), for comparison, does undergo
photodissociation (7) and also has LMCT bands in the near-
UV and ligand field (LF) bands at lower energy, the coupling
between LMCT and LF bands is quite strong in the osmium
complex (19). As a consequence one expects internal conver-
sion from LMCT to LF states to be considerably more rapid in
OsCl62), so that the lifetimes of the LMCT states would be
shorter and the excited states correspondingly less reactive.
When ethanol was not completely removed from the
chloroform, a photosubstitution reaction took place, yielding
OsCl5(C2H5OH)), verified from its UV spectrum (31). Photo-
substitution of chloride in OsCl62) by one solvent molecule has
been observed in coordinating solvents (32). While photosub-
stitution was taking place, the rate of HCl production
decreased, reaching zero with complete substitution. Likewise,
traces of water in the chloroform led to photosubstitution to
form, as verified by its UV spectrum (31,32), OsCl5(H2O)).
It too was photocatalytically inert.
14. Wilputte-Steinert, L. (1978) Photoinduced catalysis—Hydrogen
transfer reactions catalyzed by ferrocene and ⁄ or ferric chloride.
J. Mol. Catal. 4, 113–123.
15. Gasyna, Z., W. R. Browett and M. J. Stillman (1984) One-elec-
tron, visible-light photooxidation of porphyrins in alkyl chloride
solutions. Inorg. Chem. 23, 382–384.
The failure of the aquapentaammineosmium(IV) complex
to catalyze chloroform photodecomposition is quite consistent
with the known inverse effect of coordinated water molecules
on excited state lifetimes (33), which is based on the increase in
the rate of nonradiative relaxation caused by high-frequency
O–H or N–H vibrations in coordinated groups (34). Because
of the shorter excited state lifetime for OsCl5(H2O)) compared
with OsCl62), the bimolecular rate constant for Eq. (9) can be
expected to be correspondingly lower.
˜
16. Munoz, Z., A. S. Cohen, L. M. Nguyen, T. A. McIntosh and P. E.
Hoggard (2008) Photocatalysis by tetraphenylporphyrin of the
decomposition of chloroform. Photochem. Photobiol. Sci. 7, 337–
343.
˜
17. Cohen, L. R., L. A. Pena, A. J. Seidl, J. M. Olsen, J. Wekselbaum
and P. E. Hoggard (2009) The photocatalytic decomposition of
chloroform by tetrachloroaurate(III). Monatsh. Chem. 40, 1159–
1165.
CONCLUSION
2)
18. Cohen, L. R., L. A. Pena, A. J. Seidl, K. N. Chau, B. C. Keck, P.
L. Feng and P. E. Hoggard (2009) Photocatalytic degradation of
chloroform by bis (bipyridine)dichlororuthenium(III ⁄ II). J. Co-
ord. Chem. 62, 1743–1753.
While OsCl6
has shown itself to be quite effective in
catalyzing the photodecomposition of chloroform under
near-UV irradiation, ligand exchange occurs in the presence
of even trace amounts of water, completely quenching the
catalysis. In the absence of available ligands, i.e. in pure
19. Allen, G. C., R. Al-Mobarak, G. A. M. El-Sharkawy and K. D.
Warren (1972) Electronic spectra of the hexahalo anions of
osmium(IV) and iridium(IV). Inorg. Chem. 11, 787–796.
20. Du, H., R.-C. A. Fuh, J. Li, L. A. Corkan and J. S. Lindsey (1998)
Photochem CAD: A computer-aided design and research tool in
photochemistry. Photochem. Photobiol. 68, 141–142.
21. Lindsey, J. S., PhotochemCAD spectra recorded by Junzhong Li
and Richard W. Wagner. http://omlc.ogi.edu/spectra/Photo-
chemCAD/html/index.html. Accessed November 2009.
22. Stone, A. and E. B. Fleischer (1968) The molecular and crystal
structure of porphyrin diacids. J. Am. Chem. Soc. 90, 2735–
2748.
23. Solis Montiel, E. and J. A. Solano (1986) Spectrophotometric
analysis for chlorine by the extraction of triiodide formed in
chloroform solution of tetrabutylammonium perchlorate. Inge-
nieria y Ciencia Quimica 10, 45–48.
24. Hicks, M. and J. M. Gebicki (1979) A spectrophotometric method
for the determination of lipid hydroperoxides. Anal. Biochem. 99,
249–253.
chloroform, photodecomposition occurs in a process that
2)
begins with the reduction of CHCl3 by excited state OsCl6
Because photosubstitution takes place rapidly in the presence
.
2)
of water, OsCl6 would be unsuited for halocarbon remedi-
ation in aqueous systems.
Acknowledgement—This work was supported by the National Science
Foundation through Grant CHE-0749681.
REFERENCES
1. U.S. Environmental Protection Agency (1975) Preliminary
Assessment of Suspected Carcinogens in Drinking Water [Report to
Congress]. EPA Document 560475003; 120 pp.
2. U.S. Environmental Protection Agency disinfection byproducts: A
reference resource. Available at: http://www.epa.gov/enviro/html/
icr/gloss_dbp.html. Accessed November, 2009.
3. Pavelic, P., P. J. Dillon and B. C. Nicholson (2006) Comparative
evaluation of the fate of disinfection byproducts at eight aquifer
storage and recovery sites. Environ. Sci. Technol. 40, 501–508.
4. U.S. Environmental Protection Agency. (1999) Ambient Water
Quality Criteria for Chloroform. EPA Document 440/5-80-033.
25. Horvath, A. and S. Papp (1987) Photooxidation of hexacyano-
ferrate(II) and pentacyanotributylphosphinoferrate(II) in chloro-
form. Acta Chim. Hung., 124, 765–771.
26. Ng, W. and P. E. Hoggard (2001) Kinetics of the photoreduction
of bis(2,4-pentanedionato)copper(II) in chloroform. Inorg. Chim.
Acta, 321, 1–4.
27. Traverso, O. and F. Scandola (1970) Photooxidation of ferrocene
in halocarbon solvents. Inorg. Chim. Acta, 4, 493–498.

470 Laura A. Pen˜a and Patrick E. Hoggard
28. Yao, Q. and A. W. Maverick (1986) One- and two-electron
photooxidation of a molybdenum(III) thiocyanate complex.
J. Am. Chem. Soc., 108, 5364–5365.
29. Heath, G. A., K. A. Moock, D. W. A. Sharp and L. J. Yellowlees
(1985) The orderly progression of redox behavior of 4d- and 5d-
hexachlorometallate complexes. J. Chem. Soc., Chem. Commun.
1503–1505.
30. Thornton, A. T. and G. S. Laurence (1973) Kinetics of oxidation
of transition metal ions by halogen radical anions. II. Oxidation of
cobalt(II) by dichloride ions generated by flash photolysis.
J. Chem. Soc., Dalton Trans. 1632–1636.
31. Maiboroda, A., G. Rheinwald and H. Lang (2000) Synthesis and
reaction of the novel complex [AsPh4][OsCl5(H2O)]. X-ray struc-
ture analysis of [AsPh4][OsCl5(H2O)]Æ2EtOH and [AsPh4]
[OsCl5(EtOH)]ÆEtOH. Inorg. Chem., 39, 5725–5730.
32. Hasenpusch, W. and W. Preetz (1977) Photochemical ligand
exchange of hexahaloosmates(IV). Zeitschrift fuer Anorganische
und Allgemeine Chemie 432, 107–114.
33. Supkowski, R. M. and W. D. Horrocks Jr (2002) On the deter-
mination of the number of water molecules, q, coordinated to
europium(III) ions in solution from luminescence decay lifetimes.
Inorg. Chim. Acta 340, 44–48.
34. Kuehn, K., F. Wasgestian and H. Kupka (1981) The role of
hydrogen vibrations in the radiationless deactivation of chro-
mium(III)-alkylamine complexes in their lowest doublet state.
J. Phys. Chem. 85, 665–670.