Catalytic oxidation of ascorbic acid via copper-polypyridyl complex immobilized on glass

Article abstract of DOI:10.1039/c4ra00817k

A monolayer of redox-active copper-polypyridyl complexes was prepared on glass and silicon substrates. The monolayer was characterized in detail by a combination of complementary surface analysis techniques. The immobilized complex was utilized for catalytic oxidation of ascorbic acid. The surface bound catalyst was found to be temporally and thermally stable, efficient, as well as partially recyclable. It also showed marked improvement over the performance of the same complex under homogeneous conditions, in terms of the catalytic activity and turnover numbers. Such supported catalyst systems are drawing significant attention as they enable merging of the virtues of homogeneous and heterogeneous catalysis.

Full text of DOI:10.1039/c4ra00817k

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PAPER
Catalytic oxidation of ascorbic acid via copper–
polypyridyl complex immobilized on glass†‡
Cite this: RSC Adv., 2014, 4, 23168
Vikram Singh, Prakash Chandra Mondal,ab Megha Chhatwal,a
a
*
c
c
*
Yekkoni Lakshmanan Jeyachandran and Michael Zharnikov
A monolayer of redox-active copper–polypyridyl complexes was prepared on glass and silicon
substrates. The monolayer was characterized in detail by a combination of complementary surface
analysis techniques. The immobilized complex was utilized for catalytic oxidation of ascorbic acid. The
surface bound catalyst was found to be temporally and thermally stable, efficient, as well as partially
recyclable. It also showed marked improvement over the performance of the same complex under
homogeneous conditions, in terms of the catalytic activity and turnover numbers. Such supported
catalyst systems are drawing significant attention as they enable merging of the virtues of
homogeneous and heterogeneous catalysis.
Received 28th January 2014
Accepted 13th May 2014
DOI: 10.1039/c4ra00817k
www.rsc.org/advances
an assembly can be easily combined with a microuidic
system, which simplies the experiments.
1. Introduction
Homogeneous and heterogeneous catalytic systems
frequently face recovery problems and suffer from low yield and
limited reusability.28–30 In contrast, analogous self-assembled
monolayer (SAM) based systems offer customized design, ease of
recovery and recyclability, in addition to marked stability and
superior activity through “acceleration effects” as exemplied by
typically large turnover numbers.31 This has been demonstrated
in several experiments in which SAMs containing metallo–ligand
complexes were used for catalysis. In particular, functionalized
rhodium phosphanes on gold32 and palladium complexes on
silicon33 were used for catalyzing hydrosilylation of alcohols and
aerobic oxidation of benzylic alcohols, respectively. In another
example, a palladium surface covered with alkanethiol SAMs
showed enhanced regioselectivity towards the reduction of
unsaturated epoxides.34 Further, a Fe-phthalocyanine monolayer
on Ag (110) surface has been utilized for oxygen reduction reac-
tion.35 Signicantly, functional SAMs can potentially unite the
advantages of homogeneous, heterogeneous, and enzymatic
catalysis at a single platform, including heterogenization of the
homogeneous catalyst through structural manoeuvres.16,31,36,37
A relevant issue in this context is the oxidation of ascorbic
acid (vitamin C, AA) which is a well known biochemical reac-
tion.38,39 Note that AA is an organic acid soluble in water. It
oxidizes to dehydroascorbic acid (DHA) which permeates easily
through blood–brain barrier where it quickly reduces back to
AA, raising the brain antioxidant level.40 In its turn, DHA is
known to have strong antiviral capability against three different
classes of viruses, viz. herpes simplex virus type 1 (HSV-1),
inuenza virus type A, and poliovirus type 1. Accordingly, the
degree of cytotoxicity of the DHA against these viruses has been
found to be 10-fold of AA.41,42 Additionally, DHA was found to
Surface assembly of polypyridyl complexes of transition
metals by both top-down and bottom-up fabricating method-
ologies is an active area of research,1–3 as it presents an
opportunity to incisively engineer and combine both molec-
ular and surface attributes, adjusting chemical, optical,
structural, magnetic and electrochemical properties of the
entire system.4–8 Besides, this platform has signicant poten-
tial applicability in logic operations,9,10 sensing,11,12 drug
delivery,13 articial nuclease activity,14,15 and catalysis.16
Among these issues, catalysis mediated by supported molec-
ular lms is a largely unexplored eld. Potentially, a proper
immobilization of suitable molecules on a solid support,
including nanoparticles (NPs),17–22 should result in the desired
exposure of the receptors or active sites toward the reac-
tants.23–25 Whereas NP supported catalytic systems have
certainly more potential for practical applications, model
studies, dealing with the general methodology of the
approach, should be better performed on a at support, which
allow better control of the experiments and easier analysis of
their results.26 Generally, molecular assembly on at surfaces
is considered as an excellent platform for studying and nding
out mechanistic details of reactions at interfaces.27 Also, such
aDepartment of Chemistry, University of Delhi, Delhi-110007, India. E-mail: vikram.
singh19@yahoo.co.in
bDepartment of Chemical Physics, Weizmann Institute of Science, Rehovot-7610001,
Israel
cApplied Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253,
69120 Heidelberg, Germany. E-mail: Michael.Zharnikov@urz.uni-heidelberg.de
† Dedicated to Late Dr Tarkeshwar Gupta.
‡ Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra00817k
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2.2 Fabrication of monolayer of complex 1 (ref. 51)
The freshly cleaned glass and Si substrates (20 mm ꢁ 10 mm ꢁ
1 mm) were functionalized with 3-iodo-n-propyltrimethoxy-
silane under N2 atmosphere using glove box. The substrates
were initially treated with a dry n-pentane solution of 3-iodo-n-
propyltrimethoxy-silane (200 : 1, v/v) at room temperature for
30 min under N2 atmosphere. Then the substrates were thor-
oughly washed with dry n-pentane and sonicated (for 3 min
each) with n-pentane, followed by washing with dichloro-
methane (DCM) and 2-propanol to remove any physisorbed
materials. The resulting lms were dried properly under a
ꢀ
stream of N2 followed by drying in oven at 120 C for 15 min.
Scheme
1 A schematic representation of fabrication of 1-based
Consequently, the functionalized substrates were placed into
50 mL Teon-lined autoclave, immersed in dry acetonitrile–
toluene solution (3 : 7, v/v) of complex 1 (0.5 mM), and then kept
monolayer on glass or Si support. (i) Immobilization of 3-iodo-n-
propyl-trimethoxysilane to form a coupling layer. (ii) Quaternization of
the pendant pyridyl group of 1 to form an 1-based monolayer with the
acetate moiety termination.
ꢀ
at 85 C for 52 h in a programmed oven. The oven was subse-
quently cooled to room temperature. The resulting, functional-
ized substrates were rinsed with acetonitrile and sonicated for 3
min each in acetonitrile, acetone, and 2-propanol to remove
physisorbed materials. Finally, the samples were carefully wiped
with a wet task wipe and dried under a stream of N2.
inhibit enzymatic activity of IkB kinase, resulting in inhibiting
NF-kB activation; thus, potentially playing an important role in
decreasing inammation and perhaps oncogenesis, much like
salicylate and aspirin.43
In this context, keeping in mind the involvement of DHA in
the biochemical processes and prospects of SAM based catalysis
in general, we report here a detail study on the oxidation of AA
by a supported (glass), siloxane anchored monolayer of custom
designed copper complexes (1) which have pyridine as a
pendant group (see Scheme 1). We took advantage of the exible
molecular design of such complexes that exhibit oxidation-
state-dependent geometry variations. To the best of our
knowledge, this is the rst study of SAM based oxidation of AA,
as other notable reports focused mainly on the detection of AA,
particularly, on electrode surface44,45 and nanoparticles,46,47
utilizing the oxidation–reduction behaviour of AA. Moreover,
the present work uses an environmentally benign solvent
(water) and a recyclable substrate material (glass), which
pertains to green experimental strategy.48,49 The choice of the
glass substrate over other possible supports was intentional,
since glass is inexpensive and optically transparent material
that allows robust, siloxane-based covalent attachment of the
copper complexes. The 1-based monolayer was well dened as
follows from its detailed characterization by X-ray photoelec-
tron spectroscopy (XPS), near edge X-ray absorption ne struc-
ture (NEXAFS) spectroscopy, atomic force microscopy (AFM),
ellipsometry, and UV-Vis spectroscopy.
2.3 Preparation of AA solution
A stock solution of 1.0 M of AA was prepared by dissolving 17.6 g
of AA in 100 mL DI water (degassed with N2) and kept in an air
tight glass vessel. This solution was further diluted to generate
specic solutions having different amounts (concentrations) of
AA as required during the experiments.
2.4 UV-Vis spectroscopy
UV-Vis spectra were recorded with JASCO UV-Vis NIR spectro-
photometer in a range of 200–600 nm in transmission mode.
The samples were placed in 3 mL quartz cuvettes (total capacity)
with 1 cm path length. DI water was used for the baseline
corrections. The total volume in cuvette was always maintained
at 2 mL. As mentioned in the previous section, the sample size,
corresponding to the active area of the catalyst, was always kept
at 2 ꢁ 1 cm2, at the monolayer coverage of the catalyst-bearing
molecules (0.41 nmol per cm2; see below).
2.5 XPS and NEXAFS spectroscopy
XPS measurements were performed using a Mg Ka X-ray source
and a LHS 11 analyzer. The spectra acquisition was carried out
in normal emission geometry with an energy resolution of
ꢂ0.9 eV. The X-ray source was operated at a power of 260 W and
positioned ꢂ1.5 cm away from the samples.
2. Experimental
2.1 Synthesis of complex 1 (ref. 50)
The NEXAFS spectroscopy measurements were performed at
Copper acetate monohydrate (0.193 g, 0.96 mmol) was dissolved the HE-SGM beamline (bending magnet) of the synchrotron
in 10 mL of methanol at room temperature. A boiling meth- storage ring BESSY II. The spectra acquisition was carried out at
anolic solution of 40-pyridyl-2,20:60,200-terpyridine (0.30 g, 0.96 the C, N, and O K-edges in the partial electron yield mode with
mmol) was added dropwise in 30 minutes span time and the retarding voltages of ꢃ150, ꢃ300, and ꢃ350 V, respectively.
ꢀ
solution was stirred for further 2 h at 60 C. The solution was Linear polarized synchrotron light with a polarization factor of
ltered when hot. The ltrate was cooled to room temperature ꢂ91% was used. The energy resolution was ꢂ0.3 eV at the C K-
and slow diffusion of diethyl ether to this solution yielded edge and somewhat lower at the N and O K-edges. The inci-
needle shaped green colour single crystals.
dence angle of the light was varied from 90ꢀ (E-vector in the
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surface plane) to 20ꢀ (E-vector nearly normal to the surface) in
steps of 10–20ꢀ to monitor the orientational order of the target
lms. This approach is based on the linear dichroism in X-ray
absorption, i.e., the strong dependence of the cross-section of
the resonant photoexcitation process on the orientation of the
electric eld vector of the linearly polarized light with respect to
the molecular orbital of interest.52 The raw spectra were
normalized to the incident photon ux by division by a spec-
trum of a clean, freshly sputtered gold sample. Further, the
spectra were reduced to the standard form by subtracting a
linear pre-edge background and normalizing to the unity edge
jump. The photon energy scale was referenced to the most
intense p* resonance of highly oriented pyrolytic graphite
(HOPG) at 285.38 eV.53
2.6 ESI-mass spectroscopy
Mass spectra of AA and its oxidized products were recorded with
Fig. 1 Si 2p (a), N 1s (b), Cu 2p (c), and I 3d (d) XP spectra of 1-based
THERMO Finnigan LCQ Advantage max ion trap mass spec- monolayer on Si(100) substrate. The filled circles represent the
trometer. For AA, 1 mg mLꢃ1 solution was prepared in N2
purged, DI water. For AA oxidation products, 500 mL of solution
experimental data; the black solid lines are the respective envelopes or
fits. The N 1s spectrum is decomposed into two components related to
coordinated N-atoms and pyridinium N+. The I 3d spectrum is
were directly taken from reaction cuvette.
decomposed into two doublets related to ionic I atom (black line) and
covalently bonded I atom (grey line).
2.7 FT-IR spectroscopy
FT-IR spectra were recorded with Perkin-Elmer FT-IR spec-
trometer in a range of 400–4000 cmꢃ1 using N2 purged DI water
these emissions, viz. 2.8, correlates well with the chemical
compositions of the immobilized complexes (the N/N+ ratio for
as medium. DI water was used for the baseline corrections.
The samples were prepared in the similar way as in the case of
ESI-MS.
1 is 3). The Cu 2p spectrum of the monolayer exhibited a
pronounced Cu 2p3/2, 1/2 doublet at 932.1 and 951.9 eV
(Fig. 1c), highlighting the expected presence of copper.58,59
Finally, the I 3d XP spectrum of the 1-based monolayer in
Fig. 1d was dominated by the 3d5/2,3/2 doublet at 620.13 eV
(I 3d5/2) which can be assigned to ionic iodine,60,61 in agreement
3. Results and discussion
3.1 Monolayer characterization
UV-Vis measurements on the fabricated monolayer showed that with the expected composition of the monolayer (Fig. 1d). This
the complexes 1 adhered strongly to the substrate surface as doublet was accompanied by a much weaker one at 622.93 eV
they could not be removed by either the “scotch tape decohe- (I 3d5/2) which can be attributed to methylene iodide (–CH2I). The
sion” test or by a stream of critical carbon dioxide (snow-jet).54 intensity ratio of the anionic and methylene doublets was found
Static aqueous contact angle measurements revealed that this to be 10 : 1, which suggests a low content of the methylene iodide
parameter changed from <20ꢀ for the freshly cleaned silicon and, consequently, a uniform character of the monolayer.
substrate to 67ꢀ (ꢄ5ꢀ) for the monolayer of 1, which is reason-
Complementary to the XPS data, NEXAFS spectra of the 1-
able since the latter is apparently more hydrophobic. Semi- based monolayer were recorded at the C, N, and O K-edges. The
contact AFM measurements on the monolayers of 1 grown on incidence angle of X-rays was varied to monitor possible
Si(100) substrates showed an essentially smooth lm surface. orientational order within the monolayer. The C K-edge spectra
The root-mean-square surface roughness, Rq, for these lms showed a characteristic, double-peak resonance at ꢂ285 eV
was ꢂ0.13 nm and peak-to-peak roughness, Ry, was ꢂ0.98 nm, typical of pyridine in the given specic terpyridine arrangement
for 500 nm ꢁ 500 nm scan areas (see Fig. S1 in the ESI‡).
(Fig. 2a).62 This pre-edge feature consists presumably of three
XPS analysis of the 1-based monolayer revealed the expected, individual p* type resonances which are partly merged
characteristic emissions from the substrate (SiOx/Si) and the together.63,64 The complex character of this feature as compared
attached molecules. In particular, the Si 2p spectrum exhibited to the single p1* resonance of benzene at 285.0 eV (ref. 56) is
two characteristic doublets at 99.15 and 102.6 eV associated mostly related to the different C 1s core level binding energies
with the bulk Si and SiOx, respectively (Fig. 1a).55 The relative for the carbon atoms in the ortho positions (C]N) and those in
intensity of the former doublet increased with the increasing the meta and para positions (C]C) of the pyridine rings.63,64 In
sampling depth of XPS, which suggests, as expected, that only addition, there are two further, distinct resonances at ꢂ287.5
the topmost part of the Si substrate was oxidized. The N 1s and ꢂ288.6 eV, overlapping with the absorption edge. The
spectrum of the monolayer exhibited two emissions at 399.4 former resonance is presumably related to the pyridine rings,
and 402.0 eV related to the pyridine (N) and pyridinium (N+) while the latter resonance can be associated with the excitation
nitrogen atoms, respectively (Fig. 1b).56,57 The intensity ratio of in the p* orbital of the terminal acetate groups.65 The N K-edge
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3.2 Catalytic activity of the monolayer
1-Based catalytic oxidation of AA was monitored optically using
an off-the-shelf UV-Vis NIR spectrophotometer; the measure-
ments were performed in a range of 200–600 nm, in trans-
mission mode. At rst, a reference UV-Vis spectrum of 300 nmol
of AA dissolved in 2 mL of DI water (corresponding to 150 mM
concentration) was recorded; this spectrum exhibited an
intense absorption band with lmax ¼ 264 nm. Then, the same
amount of the fresh AA solution was brought in contact with the
1-based monolayer on a 2 ꢁ 1 cm2 support, and the 1-catalysed
reaction was monitored by UV-Vis spectroscopy. The spectra
were acquired subsequently for about 4 hours with no stirring
and at room temperature. It was then observed that the inten-
sity of the characteristic absorbance band, representative of the
concentration of the AA solution, decreased gradually with the
time until it disappeared almost completely aer 4 hours
(Fig. 3). When the reaction was performed for a longer time, an
absorption band of low intensity at lmax ¼ 288 nm appeared,
implying that species other than AA were present. Most likely
these are oxidation products of AA such as DHA (oxidation
products of AA absorb very little in the given spectral region).70,71
Within this process, neutral form of AA, at rst, dissociated
reversibly to its monoionic form (ascorbate anion), which was a
slow step. Subsequently, ascorbate anion started a series of
reaction at liquid–solid interface by coordinating to Cu(II),
which eventually led to the AA oxidation.72,73
Fig. 2 C K-edge (a) and N K-edge (b) NEXAFS spectra of 1-based
monolayer on Si(100) substrate. The spectra were acquired at X-ray
incident angles of 90ꢀ, 55ꢀ, and 20ꢀ. There are no noticeable changes
in the intensity of the characteristic resonances upon the variation of
the acquisition geometry.
spectra are dominated by a single p* resonance at ꢂ399.6 eV
assigned to the excitation of the N 1s core level electron into the
LUMO orbital of the pyridine rings (Fig. 2b).64 The O K-edge
spectra are dominated by an intense p* resonances at ꢂ537 eV
accompanied by several s* resonances at higher photon ener-
gies (see Fig. S2 in the ESI‡).65 The NEXAFS spectra at both C
and K edge did not exhibit noticeable changes in the intensity of
the characteristic absorption resonances upon the variation of
the incidence angle of X-rays (see the respective difference
spectra as Fig. S3 in the ESI‡). This means that the complexes 1
did not exhibit a preferable orientation in the monolayer.66
The UV-Vis absorbance spectrum of the 1-based monolayer
on glass showed a characteristic ligand-to-metal charge transfer
band at lmax ¼ ꢂ356 nm. This spectrum was similar to the
analogous spectrum of 1 in solution but had a red shi of the
absorption band position by ꢂ8 nm (see Fig. S4 in the ESI‡).67
Increasing the reaction time from 52 to 64 hours did not affect
the optical absorption or the position of the band, indicating
the completeness of the monolayer formation within the given
preparation time. In contrast, shortening the reaction time
from 52 to 20 hours resulted in decrease in optical absorption of
the 1-based monolayer by a factor of ꢂ4. Interestingly, the
estimated volume-per-molecule for 1 on the glass surface, V ¼
0.4–0.6 nm3, corresponds approximately to the respective
value for complex 1 in the unit cell of the crystal structure, V ¼
1.1 nm3 with two molecules in the unit cell as observed in our
previous work.68 The ellipsometry derived thickness of the 1-
based monolayer was ꢂ1.4 ꢄ 0.3 nm, and the refractive index, n,
at l ¼ 550 nm (non-absorbing region) was 1.66. Note that the
above thickness value is in good agreement with the XPS
derived one (ꢂ1.2 nm). The molecular density in the monolayer
To verify the fact that the observed change of the UV-Vis
spectra was primarily due to the monolayer based oxidation of
AA and not due to water or glass slide, control measurements
with no glass slide and a blank glass slide (2 ꢁ 1 cm2) were
performed at the same conditions as the experiments with the
functionalized slide. No signicant changes in the UV-Vis
spectra were observed, which suggests that the changes occur-
ring in the case of the 1-based monolayer (Fig. 3) can be indeed
attributed to the oxidation–reduction reaction between the
Fig. 3 Subsequently acquired UV-Vis absorption spectra of AA solu-
tion (150 mM) exposed to the 1-based monolayer from the beginning of
the reaction (t ¼ 0; top curve; highlighted by thick black line) to t ¼ ꢂ4
hours (bottom curve; highlighted by thick black line). Inset shows the
appearance of a new band of very low intensity with lmax ¼ 288 nm
upon a prolonged exposure (t > 4 hours). The oxidation products of AA,
viz. DHA and DKG do not exhibit intense absorption bands in the given
spectral range.
2
2
˚
was estimated at ꢂ40 A per molecule or 0.41 nmol per cm by
UV-Vis spectroscopy; the above values are reasonable for such
species.68,69
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Table 1 Parameters of the catalytic reaction mediated by the 1-based
monolayer (2 cm2, 0.41 nmol cmꢃ2) for different concentrations of AA
in N2 purged, DI water
Entry
[AA]/nmol
S/C
Conversiona %
TONa
1
2
3
4
5
6
60
100
300
2000
10 000
100 000
150
240
730
4880
24 390
243 900
98(25)b
95(28)
92(26)
51(15)
26(5)
147(37)
235(67)
670(189)
2490(732)
6340(1219)
31 700(9756)
13(4)
a
Aer
4 h of reaction as determined by UV-Vis spectroscopy.
b
Conversion % and TONs in the brackets correspond to the reaction
with the homogeneous 1-based catalytic system (solution study) under
the similar conditions.
Fig. 4 Change in the AA concentration upon exposure of its fresh
solution in water to 1-based monolayer (black bars) or to blank glass
slide (light grey bars) or just keeping it (grey bars) for ꢂ4 hours. The
change in the AA concentration was only ꢂ4–5% in two latter cases
but amounted ꢂ98% at the exposure to the monolayer. Inset: semi-
logarithmic plot of the AA concentration as a function of time, along
with the respective linear fit (R2 ¼ 0.99) suggesting the first order
kinetics for the 1-catalyzed oxidation of AA.
numbers (see Table 1). In particular, 100 mmol of AA dissolved
in 2 mL of DI water (corresponding to 50 mmol concentration)
with a S/C of 243 900 were exposed to the 1-based monolayer
(2 ꢁ 1 cm2). As expected, we observed that in the time-frame of
ꢂ4 hours, there was only 13% conversion but this still provides
a high turnover number of 31 700. In addition, we also
compared the catalytic activity of the 1-based monolayer with
Cu(II) complex in solution (homogeneous catalysis) (MeOH–
H2O mixture, 1 : 1 (v/v)). The homogeneous catalysis demon-
strated an inferior efficiency with low conversion percents at
various S/C ratios under the similar conditions. For instance,
60 nmol of AA provided only ꢂ25% conversion (TON ¼ 37),
which is signicantly lower than ꢂ98% (TON ¼ 147) as observed
for the monolayer based catalysis. Similar differences in reac-
tion yield were also noticed for the higher amounts of AA at the
same S/C ratios (see Table 1). The observed lower yields for the
homogeneous catalysis are in agreement with the previously
reported results by Fernandes et al.,31 Sawamura et al.32,33 and
Benitez et al.26 and can be tentatively explained by partial
aggregation of the catalyst molecules in solution, leading to
their deactivation. In contrast, according to Belser et al.,17 in the
case of heterogeneous catalysis, aggregation of the catalyst
molecules is rather unlikely due to the separation of the cata-
lytic centers upon the immobilization, which improve reaction
Cu(II) centre of the monolayer and AA. To stress the role of this
centre, we added copper metal (Cu (0)) to AA solution for ꢂ4
hours in another set of control experiments. Similar to the case
of the blank glass slide, no signicant changes in the UV-Vis
spectra were observed, which imply that Cu(II) oxidation state is
necessary for carrying out the reaction.
To make a quantitative comparison, a plot of the AA
conversion in percent with respect to time (in seconds) was
made (Fig. 4). According to this plot there is just a ꢂ4–6%
decline in the absorbance of the AA solution in the cases of the
blank glass slide and water without glass slide, while the
conversion for the 1-based monolayer approached ꢂ97% aer 4
hours. The plot of –log[AA] vs. time (in seconds) gives a straight
line, suggesting a rst order reaction for the 1-mediated
oxidation of AA. A linear t of the experimental data according
to the rst order reaction equation
[AA] ¼ [AA]initial ꢁ exp(ꢃk1T)
(1)
gives a rate constant, k1, of ꢂ2.82 ꢁ 10ꢃ4 sꢃ1
.
In further experiments we varied the amount of AA partici- yields. In addition, immobilization of the catalyst allows
pating in the reaction, dissolving 60 to 2000 nmol of AA in 2 mL favorable orientation of these catalytic centers, which is
of DI water, which corresponds to a concentration range of 30 to suggested to be a further factor in improving reaction yields.18,27
1000 mM and to a substrate to catalyst ratio (S/C) of 40 to 650 for Our results, therefore, can be tentatively explained along
the given 1-functionalized slide area (2 cm2). The resulting these lines.
solutions were subjected individually to the 1-based oxidation
In another set of experiments, we tested the lifetime and
for 4 hours. As expected, small amounts of AA (60–300 nmol) got reusability of the 1-based monolayer catalyst. For this purpose,
oxidised almost completely, resulting in high turnover numbers the monolayer was exposed four times, in a successive fashion,
approaching the S/C ratios (see Table 1). For the larger amount to either 60, 100, or 300 nmol of AA dissolved in 2 mL of DI
(2000 nmol), the oxidation was not complete and, consequently, water. Each exposure (run) took 4 h and the amount of the
the turnover number was signicantly smaller than the S/C residual AA was monitored optically. Aer each exposure, the
ratio. The respective experiments were repeated two more times catalyst was regenerated by keeping it in air/O2 for ꢂ5 minutes.
with different glass slides of the 1-based monolayer of the same Independent of the amount of AA, we could observe a high
area; the extent of the AA conversion was essentially the same, conversion yield in the rst run, but the catalytic activity
within an error of ꢄ2%, which justies the results. In addition reduced noticeably with each subsequent run. The extent of the
to the above study, we explored the impact of much higher reduction was larger with increasing amount of AA in the
concentrations of AA on conversion efficiency and turnover solution as shown in Fig. 5. Such a reduction in the catalytic
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higher concentrations of AA. For instance, whereas the surface
density of the pristine monolayer was ꢂ0.41 nmol per cm2, it
dropped down to 0.28 nmol per cm2 aer four runs of exposure
to 300 nmol of AA (Fig. 6a). This plunge in the surface density
can be presumably attributed to loss of acetate ions during the
reaction with concomitant geometry change around the metal
centre or to possible contamination of the surface.68 Note that
the leaching of copper from the monolayer could be possibly
neglected as the rationale for the observed drop in the catalytic
activity as the LMCT bands at lmax ¼ ꢂ356 nm exhibited only a
slight decrease in its absorbance. To substantiate our claim, we
kept the monolayer in saturated sodium acetate solution (O2
saturated) for ꢂ10 minutes aer each run (160 nmol AA) per-
formed this time for ꢂ2 hours and it was found that the
conversion decreased to ꢂ31% aer four runs (Fig. 6c), which is
a more substantial drop as compared to the air/O2 regeneration
of the monolayer (Fig. 5). This puts into perspective the
supposed scenario of acetate ions getting drained into the
solution. On this basis, a probable mechanism is proposed for
the AA oxidation (see also Fig. S5 in the ESI‡), where AA forms
ascorbate anion which acts on Cu(II) resulting in release of
acetate ion. Electron transfer from ascorbate moiety to Cu(II)
reduces it to Cu(I) and oxidizes AA to DHA. During regeneration
of the catalyst, 1-based monolayer is kept in saturated solution
of sodium acetate (O2 saturated), where O2 oxidizes Cu(I) back to
Cu(II). Presence of acetate ions probably favours the regenera-
tion of the original catalyst.68,72,73
Fig. 5 A reusability test for the 1-based monolayer catalyst. The
monolayer was exposed four times, in a successive fashion, to either
60 nmol (left panel), 100 nmol (middle panel) or 300 nmol (right panel)
of AA dissolved in 2 mL of DI water. Each run took 4 h. The catalyst was
reset with air/O2 between the runs. After the fourth run, conversion
dropped to ꢂ65% and ꢂ12% for 60 and 300 nmol of AA, respectively.
ability could be related to subtle equilibrium between the
catalytic transformations and successive deactivation of the
catalyst.
To rationalise the above observations, UV-Vis spectra of the
monolayer were recorded aer the regeneration and it was
found that the monolayer absorbance declined per run. More-
over, evaluation of the surface density of 1 from the UV-Vis data,
on the basis of the characteristic ligand band at lmax ¼ 278 nm,
revealed that this density decreased faster aer each run for
3.3 Product analysis
A qualitative analysis of the oxidation products of AA was per-
formed by a combination of electrospray ionization mass
spectroscopy (ESI-MS, +ve mode) and FT-IR spectroscopy. The
ESI-MS analysis of AA and its oxidation products in N2 purged,
DI water validated the reasoning and arguments made in the
above sections. AA and its oxidation products can form sodium
adducts in ESI-MS and this is what we have primarily observed.
Interestingly, in the mass spectrum of AA (see Fig. S6 in the
ESI‡),74 AA appeared in dimeric form as the most abundant ion
at m/z ¼ 375 (M + Na)+ while the respective monomer appeared
at m/z ¼ 199 (M + Na)+ with lower intensity, accompanied by
even weaker peak at m/z ¼ 198 and a peak at m/z ¼ 177 assigned
to AA without sodium adduct (M + H)+. The peak at m/z ¼ 198
could be possibly attributed to a keto form of AA, formed due to
the loss of H+ ion from one of the enolic hydroxyls. It appears
that the keto form is less stable than the enolic form of AA as the
intensity at m/z ¼ 199 is higher than at m/z ¼ 198. In the mass
spectrum of the same solution aer the AA oxidation via 1-based
monolayer, new peaks at m/z ¼ 192, 214, and 215 were observed
(see Fig. S7 in the ESI‡); they could be attributed to hydrated
bicyclic hemiketal of DHA or DKG (M, M + Na)+, respectively.75–77
Fig. 6 (a) Surface density of the 1-based monolayer exposed four
times, in successive fashion, to 60 (black line), 100 (light grey line), or In addition, low intense peaks were observed at m/z of 349 (M +
H)+ and 365 and ascribed to the formation of dimeric DHA and
hydrated dimeric DHA, respectively. Expectedly, it was observed
300 (grey line) nmol of AA, measured after each run by UV-Vis spec-
troscopy. (b) Representative UV-Vis spectra of the 1-based monolayer
after the end of 4th run for the case of 60 nmol AA exposure. (c) The
change in the conversion of the oxidation reaction in the case of the
regeneration with O2 saturated solution of NaOAc. The duration of the
AA exposure (160 nmol) in each run was 2 hours.
that the peaks at m/z ¼ 375 and m/z ¼ 199 disappeared
completely, emphasizing the fact that AA has been oxidised.
Thus, the ESI-MS results clearly indicate that the catalytic
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products of the catalytic reaction were analyzed. The latter
experiments relied on the easy characterization of the reaction
products, which was possible in the given case since the
monolayer catalyst was easy to remove from the reaction
mixture by facile physical transfer.
The above results provide a representative example for the
usefulness of monomolecular catalytic assemblies on a solid
support. There are, however, still a variety of open questions
such as the structure–property relationship in these systems,
better reusability, etc. One can also look forward to the possi-
bility to design multi-component monolayers of catalytically
active molecules which, due to a synergy effect, could possibly
exhibit enhanced catalytic activity compared to the individual
species.
Fig. 7 (a) Transmission FT-IR spectrum of AA in N2 purged, DI water;
the spectrum show characteristic vibrations in a region of 400–3600
cmꢃ1. (b) Transmission FT-IR spectrum of the same solution after the
AA oxidation catalysis. See text for details. The units of the Y-axis are %
transmission.
Acknowledgements
This work was nancially supported by the Department of
Science and Technology (SR/S1/IC-19/2009), New Delhi, and the
University Grants Commission (F-37-407/2009(SR)), New Delhi.
VS thanks UGC for the senior research fellowship. PCM thanks
CSIR for senior research fellowship. MC thanks DAE-BRNS for
research fellowship. MZ thanks Ch. Woll and A. Nefedov (KIT)
for the technical cooperation at BESSY II and the BESSY II staff
for the assistance during the experiments.
system of the 1-based monolayer effectively oxidizes AA to DHA
and DKG in their hydrated, dimeric or hemiketal forms.
Signicantly, ESI-MS did not show any peak corresponding to
leaching of 1 or large fragments from the monolayer, which
means that the analyzed solutions were free from catalyst-
related contamination.
¨
Complimentary to the ESI-MS studies, FT-IR spectroscopy
was used to investigate the interactions between the involved
species and changes in the chemical compositions, that is, from
the reactant to product. FT-IR spectra of AA and its oxidation
products taken from the same solution (AA in N2 purged, DI
water) before and aer the catalytic process, respectively, reveal
the expected picture.78–81 In the FT-IR spectra of AA in Fig. 7a,
peaks in a region of 3000–3500 cmꢃ1 could be tentatively
designated to the hydroxyl groups; peaks at 1669 and 1327 cmꢃ1
correspond to carbon–carbon double bond and enol-hydroxyl,
respectively, and match with the reported FT-IR spectra of
AA.78–81 In the FT-IR spectrum of the oxidation products in
Fig. 7b, peaks at 3320, 1718, 1655, and 1636 cmꢃ1 are tentatively
assigned to hydroxyl, oxidated ester carbonyl groups, and
conjugated carbonyl groups corresponding to DHA and
DKG, respectively. Further peaks at 1266 and 1029 cmꢃ1 are
attributed to the carbon–oxygen stretching of the acid and ether
groups of DHA.
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