Hydrogen generation from water upon CpMn(CO)3 irradiation in a hexane/water biphasic system

Article abstract of DOI:10.1021/om1010403

Photolysis of CpMn(CO)₃ in a hexane/water biphasic system has been shown to generate hydrogen peroxide and hydrogen in 40-50% yield. Photolysis of the title compound results in loss of a CO ligand followed by coordination of a water molecule. The initially formed CpMn(CO) ₂(H₂O) intermediate was detected using time-resolved IR spectroscopy. The cyclopentadiene (CpH) monomer is generated as the major product, formed by the transfer of an H atom from the coordinated H₂O solvent to the Cp ring. A simple mechanism for H₂ generation is proposed on the basis of deuteration studies which demonstrate the production of D₂ and CpD upon photolysis of CpMn(CO)₃ in a hexane/D₂O biphasic system.

Full text of DOI:10.1021/om1010403

ARTICLE
pubs.acs.org/Organometallics
Hydrogen Generation from Water upon CpMn(CO)3 Irradiation
in a Hexane/Water Biphasic System
Jun Wei Kee,‡ Yong Yao Tan,‡ Bert H. G. Swennenhuis,† Ashfaq A. Bengali,† and Wai Yip Fan*,‡
†Department of Chemistry, Texas A&M University at Qatar, PO Box 23874, Doha, Qatar
‡Department of Chemistry, National University of Singapore, 3 Science Drive, Singapore 117543
ABSTRACT: Photolysis of CpMn(CO)3 in a hexane/water
biphasic system has been shown to generate hydrogen peroxide
and hydrogen in 40ꢀ50% yield. Photolysis of the title com-
pound results in loss of a CO ligand followed by coordination of
a water molecule. The initially formed CpMn(CO)2(H2O)
intermediate was detected using time-resolved IR spectroscopy. The cyclopentadiene (CpH) monomer is generated as the major
product, formed by the transfer of an H atom from the coordinated H2O solvent to the Cp ring. A simple mechanism for H2
generation is proposed on the basis of deuteration studies which demonstrate the production of D2 and CpD upon photolysis of
CpMn(CO)3 in a hexane/D2O biphasic system.
’ INTRODUCTION
2029 and 1947 cmꢀ1. The solution was photolyzed until there
were no further absorption changes in the IR spectrum. Mass
spectrometric analysis of the headspace above the solution
demonstrated the presence of hydrogen gas, as evidenced by a
signal at m/e 2 (Figure 1). Since the photolyzed solution was
frozen using liquid nitrogen prior to sampling, the signal is
primarily due to H2 gas produced as a result of the photoreaction
and is not a result of hexane or water vapor sampled above the
solution. Calibrating against a fixed pressure of pure H2, the yield
of H2 produced from CpMn(CO)3 irradiation was determined to
be 47 ( 12% with respect to the initial mole quantity of CpMn-
(CO)3. The time evolution of H2 demonstrates that its produc-
tionslows downconsiderably toward the end ofirradiation, due to
depletion of the parent tricarbonyl. When D2O was used instead
of H2O, D2 was the primary gaseous product, as evidenced by a
peak at m/e 4 in the mass spectrum. The mass spectrum did not
provide any evidence for the formation of O2 and CO2 as a result
of this photoreaction.
In control experiments, photolysis of CpMn(CO)3 in dried
hexane yielded very small amounts of H2 (<5% yield). No
hydrogen was detected in the absence of photolysis or when
the reaction mixture was photolyzed under air exposure. How-
ever, addition of water to the dried hexane solution restored the
hydrogen signals upon irradiation. In addition, the H2 yield
remained unchanged upon addition of 1ꢀ5 mol equiv of various
acids (HBF4, CH3COOH, HPF6) to the solution. Together with
the control experiments, the data lead to the important conclu-
sion that both H2O and CpMn(CO)3 are necessary for the
production of H2 gas in this photoprocess.
There has been considerable interest in the generation of
hydrogen gas by the photoactivation of water using transition-
metal complexes. A detailed understanding of the photopro-
cesses involved may further the “green” goal of using hydrogen as
a major energy source, thereby reducing the dependence on fossil
fuels. Several examples of metal complexes are known to activate
water, and some of them contain platinum,1,2 cobalt in
cobaloximes,3,4 and iron in hydrogenase mimics.5ꢀ8 In addition,
the discovery of a ruthenium metal complex which reacts with
water leading to consecutive thermal H2 and light-induced O2
production from water has recently been reported by Kohl et al.9
Iron carbonyl has been used together with an iridium photo-
sensitizer and sacrificial reagents to generate hydrogen from
proton sources.10 Recently, a molybdenumꢀoxo catalyst has also
been shown to generate hydrogen from water.11 The mechanism
behind water activation by metal complexes has been highlighted
in a recent review.12 In some instances, oxidative addition of
water to a metal center may be part of the overall transformation
for the reduction of H2O to yield H2. The importance of water
photoactivation by transition-metal centers necessitates more
extensive studies aimed at understanding the various photopath-
ways that may lead to hydrogen generation from water, especially
under ambient conditions. In this paper, we report the results of a
study which demonstrates the production of hydrogen and
hydrogen peroxide from water at room temperature by UV
irradiation of CpMn(CO)3 (Cp = η5-C5H5) in a hexane/water
biphasic system. A mechanism to account for this photoprocess is
also proposed.
In addition to H2, hydrogen peroxide was also detected in the
aqueous layer upon CpMn(CO)3 photolysis. Three different
assays, including the Fenton reagent test, were conducted to
’ RESULTS
H2 and H2O2 Production. Broad-band irradiation
(300ꢀ800 nm) of CpMn(CO)3 in a hexane/water two-phase
system results in the disappearance of the parent νCO bands at
Received: November 4, 2010
Published: March 23, 2011
r
2011 American Chemical Society
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Figure 2. UVꢀvis spectra showing the absorption maximum at 420 nm
of 9.29 ꢁ 10ꢀ4 M K3Fe(CN)6: (a) before addition; (b) after addition of
H2O2 produced from 20 min of photolysis, corresponding to 0.95 ꢁ
10ꢀ4 M H2O2; (c) after addition of H2O2 from 360 min of photolysis
corresponding to 2.95 ꢁ 10ꢀ4 M H2O2.
in a >90% yield of CpH relative to the initial amount of CpMn-
(CO)3. Partially deuterated C5H5D was the main product when
CpMn(CO)3 was photolyzed in a biphasic hexane/D2O mixture.
Importantly, the CpH signals decreased dramatically (<10%
yield) when CpMn(CO)3 photolysis was carried out in dried
hexane, suggesting that the presence of water is necessary for the
production of CpH.
As shown in Figure 3, irradiation of the CpMn(CO)3 hexane/
water system resulted in the growth of four new IR-active bands,
which are assigned to CO stretching absorbances of manganese
carbonyl complexes formed upon photolysis. The intensities of
all four peaks also decreased significantly when dried hexane was
used. Since photolysis of CpMn(CO)3 is known to result in the
loss of a CO ligand,15 and by analogy with several other
characterized CpMn(CO)2L complexes, the position and the
intensities of the four peaks suggest that they correspond to two
CpMn(CO)2(η2-alkene) species,16,17 one with CO bands at
1968 and 1910 cmꢀ1 and the other at 1963 and 1905 cmꢀ1
.
Figure 1. (a) Time-dependent mass spectra taken of the headspace
content upon CpMn(CO)3 photolysis in a hexane/H2O or hexane/
D2O mixture. The m/e values of 2 and 4 corresponding to H2 and D2
were scanned over time during which the gases (H2 standard, headspace
contents of CpMn(CO)3/H2O and CpMn(CO)3/D2O after photo-
lysis) were being injected into the mass spectrometer. (b) Gas-phase
mass spectra showing the fragmentation pattern of the headspace
obtained from the hexane photolysis of CpMn(CO)3 with (i) H2O
and (ii) D2O.
The first dicarbonyl compound is identified as the CpMn(CO)2-
(η2-CpH) complex, since photolysis of CpMn(CO)3 in neat
CpH solvent yielded a species with identical CO stretching band
positions. The second dicarbonyl species could be isolated as a
yellow solid because of its poor solubility in hexane. The similar
CO band positions for the second dicarbonyl species strongly
suggest that it is also a η2-bound alkene complex and hence has
been assigned to the dimer complex CpMn(CO)2(μ-η2:η2-
CpH)CpMn(CO)2, which was observed only when the ratio
of CpMn(CO)3 to CpH was increased significantly. In addition,
the syntheses of these two manganese complexes have previously
been reported.18 In order to confirm the identities of the
complexes, we have further obtained their NMR spectra and
found very good agreement with the literature values.
As mentioned earlier, UV photolysis of CpMn(CO)3 in the
solution phase results in the dissociation of a CO ligand and
coordination of a solvent molecule to the vacant site within
picoseconds of CO loss.15 For example, CpMn(CO)2(THF) is
formed in high yield upon photolysis of CpMn(CO)3 in the
presence of THF.19 The CpMn(CO)2(OHCH3) complex has
also been observed previously upon photolysis of the tricarbonyl
confirm the presence of H2O2 (see the Experimental
Section).13,14 The yield of H2O2 production was determined by
titration with standardized acidified potassium hexacyanoferrate-
(III) (Figure 2) and the titrimetric analysis of iodine produced by
the reaction of H2O2 with acidified potassium iodide. Yields of 36
( 5% and 42 ( 4% were obtained using the Fe(CN)63- and Iꢀ
assays, respectively, resulting in an average yield of 39 ( 5% for
H2O2 production relative to the initial amount of CpMn(CO)3.
NMR and IR Spectroscopy. The 1H NMR spectrum taken of
the hexane layer following photolysis of CpMn(CO)3 was
consistent with the generation of cyclopentadiene monomer,
C5H6 (CpH). Complete photolysis of CpMn(CO)3 resulted
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Figure 3. (a) Photolysis of CpMn(CO)3 (1947, 2029 cmꢀ1) in wet
hexane, producing two sets of product peaks: CpMn(CO)2(η2-C5H6)
(A, 1910, 1968 cmꢀ1) and CpMn(CO)2(μ-η2:η2-CpH)CpMn(CO)2
(B, 1905, 1963 cmꢀ1). (b) Photolysis of CpMn(CO)3 in neat cyclo-
pentadiene, producing A. (c) Photolysis of CpMn(CO)3 in hexane
solution of cyclopentadiene (CpMn(CO)3:CpH = 2:1), producing A
and B. All spectra were recorded in hexane solvent.
Figure 5. Proposed mechanism for the generation of H2 and H2O2
from CpMn(CO)3 photolysis in a hexane/water biphasic system.
time-resolved IR spectrum obtained upon 355 nm laser photo-
lysis of CpMn(CO)3 in the hexane/water system (Figure 4),
shows the formation of a short-lived complex with τ1/2 = 0.6 s at
293 K. The relative intensities and positions of the CO stretching
absorbances for this species observed at 1862 and 1934 cmꢀ1 are
similar to those of the CpMn(CO)2(THF) complex in a
cyclohexane solution at 1860 and 1931 cmꢀ1. This short-lived
species is therefore identified as the CpMn(CO)2(OH2) com-
plex. It is interesting to note that while CpMn(CO)2(THF) has a
relatively long lifetime of several minutes at room temperature,
the water complex is significantly less stable. The lower stability
of the water complex may be due to the weaker donor character-
istics of the oxygen atom in water relative to those in THF.
However, as discussed below, it may also be due to subsequent
reactivity unique to the water complex.
The data presented above strongly suggest that water is
intimately involved in the liberation of hydrogen upon photolysis
of CpMn(CO)3 in the hexane/water system. The proposed
mechanism for this reaction has been illustrated in Figure 5.
An important intermediate in this mechanism is the water-
coordinated complex CpMn(CO)2(H2O), which has been di-
rectly observed by time-resolved IR spectroscopy. In comparison
to the more stable CpMn(CO)2(THF) complex, the shorter
lifetime of CpMn(CO)2(H2O) may be the result of a fast
intramolecular proton transfer to the η5-coordinated Cp ring.
This type of H atom transfer is not without precedence in these
systems. For example, Top et al. reported that photolysis of
CpMn(CO)3 derivatives in the presence of a proton source
results in the formation of the appropriate solvate complex
Figure 4. Difference FTIR spectrum obtained upon photolysis of
CpMn(CO)3 in a water-saturated hexane solution. The negative peaks
are due to depletion of the parent tricarbonyl upon photolysis. The
positive peaks are due to the CpMn(CO)2(H2O) complex. The inset
shows the decay of the CO stretching absorptions of the water complex
at 293 K.
in the presence of methanol.20 By analogy with these η1-oxygen
coordinated complexes, we were interested in determining
whether the CpMn(CO)2(OH2) complex was generated upon
photolysis of CpMn(CO)3 in the hexane/water system. The
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followed by intramolecular proton transfer to the Cp ring.20 The
resulting 16-electron diene manganese dicarbonyl intermediate
can subsequently react via two pathways, with the first one being
CpH ligand dissociation. This process accounts for the detection
of free CpH in the 1H NMR spectrum of the hexane layer and
also for the presence of IR signals due to CpMn(CO)2(η2-CpH).
Interestingly, one of the main species detected in the hexane/
D2O two-phase system is CpD (C5H5D) together with D2
liberation. As shown in the proposed mechanism, formation of
these two major products is only possible through coordination
of a second D2O molecule to the (η4-CpD)Mn(CO)2(OD) 16e
intermediate. Thus, D2 is formed when the endo-D of the η4-
CpD ligand combines with one of the D atoms of the coordinated
D2O molecule. The second D2O molecule has to coordinate or at
least undergo some stereospecific interaction with the diene
intermediate; otherwise, the unlikely reaction of a free D2O
molecule with the intermediate via the endo-D or exo-H atom will
lead to the production of both HD and D2 in similar quantities,
which is not observed. In a possibly concerted process, an
intermediate peroxo complex is generated together with release
of H2 or D2. The observed formation of H2O2 can then be
accounted for as a product following dissociation from the
postulated CpMn(CO)2(H2O2) complex into the aqueous layer.
Interestingly, photolysis of a M(OH)2 (M = Ru) complex was
reported to yield O2, also very likely by reductive elimination of
H2O2 followed by disproportionation.9 In a recent DFT study,
the reductive elimination of H2O2 from this ruthenium complex
has been shown to be a photolytic step because of the high
endergonicity of the reaction.21 The proposed mechanism there-
fore accounts for most of the experimental observations and
involves species that are reasonable intermediates, including the
CpMn(CO)2(OH2) complex, which is directly observed.
According to the proposed mechanism, loss of the CpH ligand
following proton transfer competes with H2 production. Thus,
addition of excess CpH to the photolysis mixture may lead to
increased H2 production. However, experiments conducted with
added CpH resulted in higher yields of the CpMn(CO)2(η2-
CpH) complex. Thisobservation is perhaps not unexpected, since
the presence of free CpH competes with the binding of water for
the vacant coordination site on the Mn center, yielding primarily
CpMn(CO)2(η2-CpH). Other attempts to improve the H2 yield
were also unsuccessful. For example, H2 production remained
unaffected when hexane was replaced by either nonpolar (e.g,
cyclohexane, benzene, toluene) or polar solvents miscible with
water (THF, DMSO, DMF, CH3OH). Indeed, use of polar
solvents resulted in reduced amounts of H2 production, possibly
due to strong binding of the polar solvent molecule to CpMn-
(CO)2, thereby preventing H2O coordination. Photolysis of
CpMn(CO)3 particles suspended in pure water also did not give
better yields of H2, while addition of CO (0.2ꢀ1 bar) into the
headspace above the mixture slowed down the generation of H2
considerably and did not lead to an increase in yield. Similarly,
varying the pH of water (5ꢀ10) has little effect on the system.
The mechanism described is similar to that proposed for the
formation of disulfides and H2 from thiols following CpMn-
(CO)3 irradiation.22 In both cases, the manganese complex
CpMn(CO)2(H2O) or CpMn(CO)2(RSH) appears upon irra-
diation. In the disulfide case, however, the process can be made
catalytic because of the more facile dissociation of the weaker
SꢀH bond and formation of the stronger SꢀS bond in compar-
ison to OꢀH and OꢀO bonds, respectively. We have also
explored the possibility of the water-gas shift reaction occurring
in the system. However, as carbon dioxide could not be detected
in the mass spectrum, we have to rule out this reaction. At the end
of irradiation, manganese deposits, possibly the oxides and
hydroxides, were observed. In order to test whether the deposits
were responsible for H2 production, they were filtered, redis-
persed in fresh water, and irradiated. However, no hydrogen was
detected in the mass spectrum.
We have proposed a mechanism that is able to explain the
general features of our experimental data: namely the production
of H2 and H2O2, the detection of CpMn(CO)2(H2O) by time-
resolved IR spectroscopy, and the formation of CpH and its
complex CpMn(CO)2(CpH) as one of the main loss reaction
pathways followed by using NMR and cw IR spectroscopy,
respectively. However, much detail about the mechanism has
not yet been investigated. For example, it is possible that the
subsequent steps suggested in Figure 5 may require further
irradiation or a MnIII(OH)2 complex actually forms before
H2O2 is expelled. We are also aware that some unknown low-
concentration manganese species in the solution or even some
insoluble deposits that have eluded detection may turn out to be
the key intermediate after all. At present, it is difficult to ascertain
the actual mechanism until much more work has been carried out
on this system.
The data presented in this study demonstrate that CpMn-
(CO)3 photolysis in a hexane/water two-phase mixture yields
both hydrogen and hydrogen peroxide from water. No other
metal complexes or proton sources are required. Since hydrogen
peroxide can be catalytically decomposed into oxygen, the
photolytic process described here can generate both H2 and
O2 at room temperature. Although the process is not yet
catalytic, we hope to stimulate interest into reviving the use of
relatively inexpensive metal carbonyl complexes of strong Lewis
acid character for photoactivating water in a stoichiometric or
catalytic manner. Further work is being carried out in our
laboratory to use CpMn(CO)3 and CpRe(CO)3 derivatives to
improve the H2 production.
’ EXPERIMENTAL SECTION
All chemicals were purchased from Sigma-Aldrich and used without
further purification, unless otherwise noted. Anhydrous hexane was triply
distilled from molecular sieves. Cyclopentadiene was obtained from the
cracking of dicyclopentadiene at 150ꢀ160 °C and used immediately
upon distillation. Chemicals used in the quantification were calibrated
against their respective primary standards. UVꢀvis absorption spectra
were recorded on a Shimadzu UV-2550 spectrometer.
Photolysis of CpMn(CO)3 in Hexane/Water Mixture. In a
typical experiment, the photolysis of CpMn(CO)3 (0.030 g, 1.5 ꢁ 10ꢀ4
mol) in hexane (5 cm3) and variable amounts of H2O (up to 5 cm3) was
conducted in an evacuated quartz apparatus (25 cm3 volume) employing
a UV broad-band lamp (wavelength range 300ꢀ800 nm, typical power
measured at 10 cm distance 0.85 W/cm2). The photolysis was carried
out over a 6 h period for completion. 1H NMR spectra were recorded on
Bruker ACF300 and AMX500 NMR spectrometers with chemical shifts
referenced to residual solvent peaks in the respective deuterated
solvents. Solution IR spectra were obtained on a Nexus 870 FT-IR
spectrometer using a CaF2 cell of 0.1 mm path length.
NMR Quantification of Cyclopentadiene. CpMn(CO)3
(0.030 g, 1.5 ꢁ 10ꢀ4 mol) in 0.5 mL of isooctane and 0.5 mL of H2O
was photolyzed for 6 h before a 0.01 mL sample was transferred into a
NMR tube containing 0.5 mL of CDCl3. The NMR spectrum of the
sample was obtained, and the quantification was performed using
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1
isooctane as an internal standard. CpH: H NMR (CDCl3) δ 6.57 m
(2H), 6.46 m (2H), 2.98 m (2H).
Fenton Reagent Test.
2Fe2þ þ 2Hþ þ H2O2 f 2Fe3þ þ 2H2O
Mass Spectrometric Determination of Hydrogen. Hydro-
gen gas was detected using a residual mass analyzer (Balzer Prisma QMS
220), where the signals at m/e 2, 3, and 4 were monitored over time. The
headspace above the CpMn(CO)3ꢀhexane/water mixture (Figure 1)
was sampled before and after photolysis. A 3 mL portion of the gas was
injected directly into the residual gas analyzer using a syringe. A sample
of pure hydrogen gas of known pressure was also injected for calibration
and hence used to calculate the amount of hydrogen present in the
photolytic mixture.
A 5 mL portion of acidified iron sulfate solution (5.09 ꢁ 10ꢀ2
M
FeSO4 in 0.05 M H2SO4 solution) was added to 0.5 mL of the aqueous
extract, and the presence of hydrogen peroxide is indicated by the
immediate color change from pale green to yellow.
Photolysis of CpMn(CO)3 under CO Pressurization. CpMn-
(CO)3 (0.030 g, 1.5 ꢁ 10ꢀ4 mol) in a mixture of 5.0 cm3 of hexane and
5.0 cm3 of H2O was photolyzed for 12 h under a CO atmosphere
(between 0.2 and 1 bar). The headspace analysis and quantitative tests
were performed subsequently.
Photolysis of CpMn(CO)3 Suspended in Water. CpMn(CO)3
(0.010 g, 5 ꢁ 10ꢀ4 mol) was ground into powder form before it was
added to 5 g of water in a quartz apparatus. The suspended CpMn(CO)3
was then subjected to UV photolysis for 6 h before the headspace above
the mixture was analyzed using mass spectrometry.
Deuteration Studies. CpMn(CO)3 (0.030 g, 1.5 ꢁ 10ꢀ4 mol) in
0.5 mL of hexane and 0.5 mL of D2O was photolyzed for 6 h before the
headspace was studied using mass spectroscopy. HD and D2 were
observed as signals at m/e 3 and 4, respectively (Figure 1b). A 0.01 mL
sample was transferred into an NMR tube containing 0.5 mL of CDCl3.
The NMR spectrum of the sample was obtained, and the integral ratio
suggests the production of CpD. A similar experiment was conducted
using 0.5 mL of d12-cyclohexane and 0.5 mL of H2O, giving only CpH
and H2 instead.
Analysis of Hydrogen Peroxide Production. CpMn(CO)3
(0.030 g, 1.5 ꢁ 10ꢀ4 mol) in a mixture of 5.0 cm3 of hexane and 5.0 cm3
of H2O was photolyzed for 6 h. The aqueous phase was removed and
subjected to the analyses below.
Photolysis of CpMn(CO)3 in Cyclopentadiene. CpMn(CO)3
(0.048 g, 1.5 ꢁ 10ꢀ4 mol) in neat cyclopentadiene (2 mL) or
cyclopentadiene in dried hexane solution (0.010 mL CpH in 2 mL of
hexane) was photolyzed for 1 h in an evacuated quartz apparatus using a
broad-band UV lamp. The solution was evaporated to dryness and the
residue redissolved in hexane. The products were characterized using IR
and NMR spectroscopy and identified to be CpMn(CO)2(η2-CpH) and
CpMn(CO)2(μ-η2:η2-CpH)CpMn(CO)2, respectively.
Alkaline Potassium Hexacyanoferrate(III).
CpMn(CO)2(η2-CpH): IR (ν(CO)) 1968 s, 1910 s cmꢀ1; 1H NMR
(C6D6) δ 6.49 m (1H), 5.47 m (1H), 4.28 m (1H), 3.88s (5H), 3.76 m
(1H), 2.95 m (2H).
2FeðCNÞ6 þ H2O2 þ 2OHꢀ f 2FeðCNÞ64ꢀ þ O2 þ 2H2O
3ꢀ
Potassium hexacyanoferrate(III) solution (9.78 ꢁ 10ꢀ3 M) was
prepared using literature procedures.14 A 19.5 mL portion of alkaline
potassium hexacyanoferrate(III) (9.78 ꢁ 10ꢀ4 M K3Fe(CN)6 in 0.4 M
KOH solution) was added to 0.5 mL of the aqueous extract from the
photolysis of CpMn(CO)3. The resulting solution was then stirred and
subjected to UVꢀvis absorption spectroscopy. The yield of H2O2 was
determined to be 36 ( 5%.
CpMn(CO)2(μ-η2:η2-CpH)CpMn(CO)2: IR (ν(CO)): 1963 s,
1
1905 s cmꢀ1; H NMR (C6D6) δ 4.72 m (1H), 3.82 s (5H), 3.18
(1H), 3.07 m (1H).
Time-Resolved Infrared Spectroscopy. A Bruker Vertex 80
instrument operating in rapid scan mode was utilized to detect the
presence of CpMn(CO)2(H2O). To a 5 mL hexane solution of 2 mM
CpMn(CO)3 was added 1 mL of distilled water. The resulting mixture
was shaken vigorously and then degassed by bubbling Ar for 5 min. After
separation, the water-saturated organic layer was extracted and added to
a 0.5 mm path length temperature controlled IR cell with CaF2 windows
(Harrick Scientific). The mixture was photolyzed with a single shot of
355 nm light from a Nd:YAG laser (Continuum Surelite I-10, 50 mJ/
pulse) and the resulting spectrum acquired at 8 cmꢀ1 resolution.
Acidified Potassium Dichromate.
Cr2O72ꢀ þ 3H2O2 þ 8Hþ f 2Cr3þ þ 7H2O þ 3O2
Potassium dichromate solution (4.08 ꢁ 10ꢀ3 M) was prepared using
literature procedures.13 A 19.5 mL portion of acidified potassium
dichromate (4.08 ꢁ 10ꢀ4 M K2Cr2O7 in 0.1 M HCl solution) was
added to 0.5 mL of the aqueous extract from the photolysis of CpMn-
(CO)3, and the resulting solution was then stirred and subjected to
UVꢀvis absorption spectroscopy. The yield of H2O2 was determined to
be 39 ( 5%.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: chmfanwy@nus.edu.sg. Fax: 65-67791691.
Acidified Potassium Iodide.
2Iꢀ þ H2O2 þ 2Hþ f I2 þ 2H2O
’ ACKNOWLEDGMENT
The project was supported by a research grants provided by
MOE Tier 2 Fund no. T208B1111 and NUS no 143000-430112.
J.W.K. acknowledges the support of an NUS scholarship.
I2 þ 2Na2SO3 f 2NaI þ Na2S4O6
Potassium iodide (5.19 ꢁ 10ꢀ2 M), sodium thiosulphate (5.16 ꢁ
10ꢀ4 M), and starch indicator solutions were prepared using literature
procedures.13 A 19.5 mL portion of acidified potassium iodide (5.19 ꢁ
10ꢀ3 M in 0.05 M H2SO4 solution) was added to 0.5 mL of the aqueous
extract, and the resulting solution was left to stand in a glass-stoppered
conical flask in the dark for 5 h until a brown coloration appeared.
The solution was then titrated with sodium thiosulfate, and 1 drop of
starch indicator was added toward the end point when the brown
coloration became too faint to be observed accurately. The end point
was defined as the point when the blue coloration of the starchꢀiodine
complex disappears. The yield of H2O2 was determined to be 42 ( 4%.
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