Increased catalytic activity of surface-immobilized palladium complexes in the fluorogenic deprotection of an alloc-derivatized coumarin

Article abstract of DOI:10.1002/chem.201102245

Catalytic surfaces have been prepared by complexation of palladium on self-assembled terpyridine monolayers on silicon. A reaction-based fluorogenic probe was developed to allow facile visualization of the catalytic potential of the surface (see figure). Superior activity of the immobilized catalyst compared with the homogeneous control reactions is demonstrated. Copyright

Full text of DOI:10.1002/chem.201102245

DOI: 10.1002/chem.201102245
Increased Catalytic Activity of Surface-Immobilized Palladium Complexes in
the Fluorogenic Deprotection of an Alloc-Derivatized Coumarin
Antony Fernandes, Peter Hensenne, Bertrand Mathy, Weiming Guo, Bernard Nysten,
Alain M. Jonas,* and Olivier Riant*[a]
Self-assembled monolayers (SAMs)[1–3] have attracted
considerable attention, as they offer the possibility of pre-
cisely engineering surface properties towards a wide scope
of applications in biology,[4] drug delivery[5] and catalysis.[6]
Functionalized SAMs consisting of metal–ligand complexes
have been developed for the immobilization of homogene-
ous catalysts on the surface of silica[7] and gold[8] particles,
but only rare examples have been performed on flat surfa-
ces. Sawamura et al. reported the remarkable activity, sub-
strate selectivity, and reusability of SAMs of thiol-derivat-
ized rhodium–phosphane complexes on gold surfaces in the
dehydrogenative silylation of alcohols.[9] Similarly, they de-
scribed the preparation of a highly efficient, palladium-func-
tionalized silicon surface from bisoxazoline SAMs for the
aerobic oxidation of benzylic alcohols.[10] Besides the im-
proved stability, easy recovery and recyclability offered by
such an approach, the unexpected superior activity of the
grafted catalysts is the main interest of the heterogenization
of homogeneous catalysts on flat substrates. Such organome-
tallic hybrid surfaces may also find tremendous potential in
asymmetric catalysis[11] and microreactor technology.[12]
nethiol SAMs showed enhanced regioselectivity towards the
reduction of an unsaturated epoxide from 11 to 94%, de-
spite a loss of 40% activity compared with the uncoated sur-
face.[17] This effect was attributed to selective binding of the
alkene moiety on the coated Pd surface at the expense of
the epoxide ring.[18] It was also demonstrated that the activi-
ty of the modified catalyst was dependent on the molecular
arrangement of the SAMs on the surface, less well-ordered
SAMs restricting access to active sites.
These studies suggest that an intricate association of fac-
tors governs the activity of surface-modified catalysts. SAMs
of transition-metal complexes can possibly afford efficient
alternatives to homogeneous catalysts regarding activity and
selectivity, allowing the design of tailored heterogeneous
catalysts.
In the aim to confirm and study in more details this accel-
eration effect, we designed a silicon surface modified with
palladium–terpyridine complexes for use in the catalytic flu-
orogenic deprotection of allyloxycarbonylACTHNUGRTENUNG(Alloc)-derivat-
ized coumarin 1 (Scheme 1), which was prepared by reaction
The acceleration effect originating from catalyst immobili-
zation is yet not clearly understood, and probable structural
and environmental parameters (density, accessibility, orien-
tation, cooperativity, interactions with the surface, and so
forth) should account for this pronounced efficiency. Some
evidence have been reported on the impact of ordering and
orientation of functionalized Langmuir–Blodgett films on
catalysis in terms of activity and selectivity.[13–15] The stability
of cobalt–porphyrin catalysts was significantly improved fol-
lowing immobilization on gold surfaces and their activity in
the catalytic oxidation of hydroquinone can possibly be re-
lated to the arrangement of the catalyst on the molecular
level.[16] Impressively, a palladium surface covered with alka-
[a] Dr. A. Fernandes, Dr. P. Hensenne, B. Mathy, Dr. W. Guo,
Prof. B. Nysten, Prof. A. M. Jonas, Prof. O. Riant
Institute of Condensed Matter and Nanosciences
Universitꢀ catholique de Louvain
Place Louis Pasteur 1
1348 Louvain-la-Neuve (Belgium)
Fax : (+32)10-47-41-68
E-mail: alain.jonas@uclouvain.be
olivier.riant@uclouvain.be
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102245.
Scheme 1. Fluorogenic deprotection of Alloc-protected coumarin 1 for
the study of Pd-functionalized silicon surface [Si-terpy-Pd].
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Chem. Eur. J. 2012, 18, 788 – 792

COMMUNICATION
of 7-hydroxy-4-methylcoumarin (2) with allylchloroformate
(Scheme 2).
Various ligand-functionalized SAMs have been developed
for the construction of heterogeneous palladium catalysts
grafted on silica.[19] Polymer-supported palladium–terpyri-
dine complexes were also recently used for carbon-carbon
coupling reactions.[20]
To easily visualize the catalytic potential of our function-
alized surfaces, we developed a simple and straightforward
fluorogenic reaction based on the Pd-catalyzed Alloc group
deprotection.[21] Fluorogenic reactions have been widely
used in biochemistry for the understanding of biological pro-
cesses.[22] Recently, fluorogenic probes were used as reac-
tion-based chemosensors for easily detecting mercury, gold,
and palladium salts.[23] In this approach, the metal-mediated
formation of a fluorescent molecule allows its direct quan-
tification. Koide et al. recently elaborated fluorescein-based
sensors allowing the quantitative detection of palladi-
um.[24–26] This elegant approach affords a powerful tool for
rapidly monitoring and optimizing reaction conditions.[27–29]
Silicon surfaces were first modified with a terpyridine-at-
tached triethoxysilane and subsequently immersed in a solu-
tion of Pd to afford the catalytic surfaces [Si-terpy-Pd]. Re-
action of 4’-(4-anilino)-2,2’:6’,2’’-terpyridine 3[30] and 3-iso-
cyanatopropyl triethoxysilane (ICTES) in THF at reflux in
presence of a catalytic amount of Et3N (Scheme 2) lead to
the formation of triethoxysilane 4.
Figure 1. Left: Typical X-ray reflectograms of a bare Si sample, a [Si-
terpy] sample and a [Si-terpy-Pd] sample. The dots are the experimental
data; the continuous lines are fits of the data. kz0 is the vertical compo-
nent of the wavevector of the incident photons in air. Right: Electron
density profiles of the samples, obtained from the fits of the reflectivity
data. The symbols are the data obtained from the separate fits of eight
[Si-terpy] and ten [Si-terpy-Pd] samples. The continuous lines are the
average electron density profiles. The dotted line is the profile of a bare
Si wafer. Plasma cleaning results in the formation of a density dip slightly
below the Si interface.
and N atoms in the X-ray photoelectron spectroscopy (XPS)
spectra supported the incorporation of the ligand. The ter-
pyridine-modified surface [Si-terpy] was then immersed in a
solution of [PdCl2ACTHNUTRGNEUNG(CH3CN)2] in CH3CN for 18 h and finally
washed with CH2Cl2. XRR analysis (Figure 1) gave an incre-
ment of average thickness of 0.5 nm and an increment in
electron content of about 350 electrons nmÀ2 (standard devi-
ation : 30 electrons nmÀ2; 10 samples), corresponding to the
addition of 0.57 nmolcmÀ2 of [PdCl
2ACTHNUTRGNEUNG(CH3CN)]. Given an
uncertainty of 10–20% on electron densities measured by
XRR, this indicates that a close to 1:1 terpy/Pd complex was
formed, with probably a small amount of uncomplexed Pd
adsorbed on the surface. XPS confirmed the incorporation
of Pd; the Pd/N ratio value of 0.28 is close to the theoretical
value corresponding to a 1:1 complex (0.17) of [Pd
ACHTUNGTERN(NUNG CH3CN)-
AHCTUNGTRENNUNG
small amount of uncomplexed Pd on the surface. However,
ICP-MS measurements eventually provided a Pd loading of
0.26 nmolcmÀ2, which is smaller than the value obtained by
treatment of the XRR data by half. Because XRR only
measures the number of electrons and relies on the compo-
sition of the layer to obtain a grafting density, and is of rela-
tively low absolute precision, the values obtained by ICP-
MS were kept for quantification; XRR was used for the rel-
ative comparison of the samples. XPS analysis of a non-
functionalized sample, obtained by immersion of a silicon
surface in the previously described Pd solution, did not
show the presence of adsorbed Pd thus indicating that the
Pd complexed (and possibly adsorbed) only onto the terpyr-
idine layer.
Scheme 2. Synthesis of fluorogenic coumarin 1, terpyridine silane 4, and
model ligand 5.
The cleaned silicon surfaces were then modified by reac-
tion with 4 in toluene at 808C for 18 h, and extensively
washed with toluene in a Soxhlet apparatus. X-ray reflectivi-
ty (XRR) indicated the formation of a neat monolayer
(Figure 1) with an average thickness of 1.5 nm, and an elec-
tron content of about 544 electrons nmÀ2 (standard deviation
: 30 electrons nmÀ2; 8 samples), corresponding to a grafting
density of approximately 0.42 nmolcmÀ2. The presence of C
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O. Riant, A. M. Jonas et al.
As already mentioned Alloc-coumarin 1 was straightfor-
wardly prepared by reaction of 7-hydroxy-4-methylcoumarin
(2) with allylchloroformate and the terpyridine model ligand
5 was prepared by reaction of 3 with benzyl isocyanate
(Scheme 2). The fluorogenic reaction was first investigated
in homogeneous conditions in presence of [PdCl2ACTHNUTRGNEN(UG CH3CN)2]
(5 mol%) and PhSiH3 (10 equiv) as the nucleophile. We
were particularly interested in finding a solvent in which
coumarin 2 is simultaneously fluorescent (for direct monitor-
ing of the reaction at low concentration) and efficiently
formed by deprotection of 1. From this preliminary study,
CH2Cl2/EtOH (1:1, v/v) appeared to be the solvent of
choice. The fluorescence spectra of both free and Alloc-pro-
tected coumarin were taken in CH2Cl2/EtOH (1:1, v/v) and
evidenced that masking the 7-OH group of methyl umbeli-
ferone 2, resulted in almost total loss of fluorescence
(Figure 2). Thus, the Pd-mediated deprotection of 1 is dis-
Figure 3. Evolution of the Pd-catalyzed deprotection of coumarin
1
(5.10À6 mol) in 100 mL CH2Cl2/EtOH (1:1, v/v) in presence of 10 equiv
PhSiH3. Conditions: a) no palladium, 5 mol% terpy; b) 5 mol% [PdCl2-
ACHUTNGREN(UNG CH3CN)2]; c) 5 mol% [PdCl2ACHUTGNTREN(NGUN CH3CN)2], 5 mol% 5; d) [PdCl2-
ACHUTNGRENUN(G CH3CN)2], S/C=600; e) [PdCl2ACHTUNGTREN(NUGN CH3CN)2], S/C=600, 5 (1/600 equiv); f)
[Si-terpy-Pd], 9.5ꢂ2.5 cm2, 0.26 nmolcmÀ2, S/C ca. 800.
Table 1. Pd-catalyzed fluorogenic deprotection of coumarin 1 (5.10À6
mol) in 100 mL CH2Cl2/EtOH (1:1, v/v) in presence of 10 equiv PhSiH3.
Entry
Catalytic system
S/C[a]
Conversion [%][b]
TON[c]
1
2
3
4
5
PdCl
PdCl
PdCl
PdCl
G
20
20
600
600
800
38
62
7
11
46
8
12
42
66
368
[Si-terpy-Pd]
[a] Substrate-to-catalyst ratio. [b] After 6 h reaction as determined by
fluorescence. [c] Turnover number.
cence, denoting that Pd is necessary for the fluorogenic re-
lease of coumarin 2.
Figure 2. Fluorescence spectra of coumarin 2 in CH2Cl2/EtOH (1:1, v/v)
(lex =326 nm) between 1.10À5 and 1.10À6 m. Fluorescence spectra of
Alloc-protected coumarin 1 (1.10À5 m) is shown in black dotted lines. The
inset shows the calibration curve of 2 in CH2Cl2/EtOH (1:1, v/v).
To assess the full potential of our surfaces, [Si-terpy-Pd]
with a 1 cm2 area was prepared and used in the catalytic flu-
orogenic deprotection of 1. We investigated the activity of
[Si-terpy-Pd] to catalyze the deprotection of increasing
quantities of 1 from 1ꢂ10À6 to 1ꢂ10À4 mol (which results in
increasing S/C ratios) in CH2Cl2/EtOH (1 mL) for 20 h reac-
tion without any stirring (Table 2).
tinctly a fluorogenic reaction which can give access to the
activity of our surfaces by following the release of fluores-
cent coumarin 2 from its non-fluorescent derivative 1.
We then used this methodology for monitoring the Pd-
mediated deprotection of 1 (5ꢂ10À6 mol, 100 mL CH2Cl2/
Table 2. [Si-terpy-Pd] (1 cm2, 0.26 nmolcmÀ2)-catalyzed fluorogenic de-
protection of coumarin 1 in CH2Cl2/EtOH (1:1, v/v) in presence of
PhSiH3 (10 equiv).
EtOH) with [PdCl2ACHTUNGTRENNUNG(CH3CN)2], [PdCl2AHCTUNGTRNE(NUGN CH3CN)2]/5 and [Si-
terpy-Pd] catalytic systems by fluorescence (Figure 3).
The homogeneous Pd ligand (5 mol%=250 nmol) reac-
tion led to 62% conversion of 1 compared to 38% for the
similar no-ligand reaction after 6 h (Table 1). When the [Si-
terpy-Pd] surface (9.5ꢂ2.5 cm2, 0.26 nmolcmÀ2, ca. 6 nmol of
Pd, S/C ca. 800) was used as the catalytic system under the
same conditions, coumarin 1 was converted in 46%, result-
ing in a turnover number (TON) of approximately 300,
which is evidently superior to the homogeneous controls
and confirmed previously reported results by Sawamura.[9,10]
When working with an S/C ratio of 600 with the homogene-
ous catalytic systems, the conversion reached only 11% in
presence of terpyridine 5 (TON ca. 66) and 7% without
ligand (TON ca. 42). The homogeneous reaction without Pd
catalyst (Figure 3a) did not show any apparition of fluores-
Entry
1
S/C
Conversion TON[a]
[%][a]
Reaction
volume [mL]
A
1
2
3
4
5
6
7
8
9
1ꢂ10À6
5ꢂ10À6
5ꢂ10À6
5ꢂ10À6
1ꢂ10À5
1ꢂ10À5
3800 84 (5)
19000 28
19000 78 (6)
19000 90
38000 23 (5)
38000 97
3200 (190)
5300
14800 (1100)
17100
8700 (1900)
36900
1
5
1
0.5
1
0.25
1
0.25
1
5ꢂ10À5 190000 14 (4)
5ꢂ10À5 190000 33
26600 (7600)
62700
1ꢂ10À4 380000
1ꢂ10À4 380000
4 (4)
9
15200 (15200)
34200
10
1
1
[a] After 20 h reaction as determined by fluorescence and H NMR spec-
troscopy; yields and TONs in brackets correspond to the conversion with
the homogeneous PdCl
tions.
2ACHTUNGTRNEN(UNG CH3CN)2/5 catalytic system under similar condi-
790
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Chem. Eur. J. 2012, 18, 788 – 792

Heterogeneous Catalysis
COMMUNICATION
As expected, the conversion decreased as S/C increased,
with a maximum conversion of 84% for S/C about 3800
(TON ca. 3200) and a minimum of 4% for S/C about
380000 (TON ca. 15200); the latter could be improved to
9% (TON ca. 34200) after 48 h reaction. Similar homogene-
ous reactions showed a different, inferior, profile with con-
versions <10% at the various S/C ratios.
We also observed that the efficiency of the catalyst was
dependent on the reaction volume. Indeed, the reaction per-
formed on 5ꢂ10À6 mol of 1 and 1 cm2 [Si-terpy-Pd] surface
was more efficient in reduced reaction volume, ranging from
28% transformation in 5 mL to 90% in 0.5 mL CH2Cl2/
EtOH (Table 2, entry 2 and 4). Reducing the reaction
volume to 0.25 mL (the minimum volume to cover the 1 cm2
surface in our setup) for the deprotection of 1ꢂ10À5 mol re-
sulted in an exceptional 97% yield (TON ca. 36900;
Table 2, entry 6). An impressive TON of about 62700 for
the reaction on 5ꢂ10À5 mol of 1 in 0.25 mL also confirmed
this tendency (Table 2, entry 8).
poor reactivity of homogeneous controls: even if all the Pd
was leached in the solution, the conversion resulting from
this low dose of free Pd would be significantly lower than
observed. In addition, it is reasonable to assume that the dif-
ferent conversions reached in the various tested conditions
result from a balance between the rate of catalytic transfor-
mation and the progressive deactivation of the catalyst. Al-
though this explains general trends, it does not explain the
increased activity of the catalyst when grafted. At this stage,
it is thus safe to conclude that the notable activity of our
catalyst dominantly arises from organization and confine-
ment of the catalyst on the surface.
In summary, a Pd-functionalized silicon surface was con-
structed by complexation on terpyridine SAMs and used in
the deprotection of an Alloc-protected coumarin. The fluo-
rogenic reaction developed here allowed a simple evaluation
and comparison between our system and homogeneous con-
trols. The surfaces showed significant superior activity com-
pared with the corresponding homogeneous reactions. Mod-
erate reusability of the heterogeneous catalyst is attributed
to Pd leaching.
To the best of our knowledge, this work represents the
second example of homogeneous catalysts immobilized on
silicon surfaces. One of the advantages of working on flat
surfaces is the possibility to pattern various motifs of SAMs
by lithographic methods, and the possibility to include these
surfaces into microfluidic chips. The fine assembling and or-
dering of catalyst-modified SAMs should provide valuable
details to understand the beneficial effect of immobilizing
and organizing homogeneous catalyst on flat surfaces. This
work is currently under investigation.
Finally, we explored the recyclability of the [Si-terpy-Pd]
surface (1 cm2) for the repeated deprotection of fluorogenic
coumarin 1 (1ꢂ10À6 mol in 1 mL CH2Cl2/EtOH, S/C ca.
3800). We observed that the activity decreased after each
new run (Table 3).
Table 3. Re-use of [Si-terpy-Pd] (1 cm2) in the catalyzed deprotection of
coumarin 1 (1ꢂ10À6 mol) in CH2Cl2/EtOH (1:1, v/v, 1 mL) in presence of
PhSiH3 (10 equiv).
Sample
Conversion
[%][a]
TON[a]
Content of layer
in electrons
[b]
[electrons nmÀ2
]
A
–
–
84
52
18
11
–
–
554
870
669
666
–
[Si-terpy-Pd]
end of 1 st run
end of 2nd run
end of 3rd run
end of 4th run
3200
2000
680
420
Acknowledgements
The authors gratefully acknowledge access to the SUCH and ASM plat-
forms of IMCN. Prof. Vincent Haufroid and Gladys Deumer are grateful-
ly acknowledged for the ICP-MS measurements. B.N. is a Senior Re-
search Associate of the FRS-FNRS. Financial support was provided by
the Communautꢀ FranÅaise de Belgique (ARC 06-11/339), the Belgian
Federal Science Policy (IAP-PAI P6/27), and the FRS-FNRS.
–
1
[a] After 20 h reaction as determined by fluorescence and H NMR spec-
troscopy. [b] Determined by XRR analysis.
After the last run of catalysis, ICP-MS analysis of the
sample gave a Pd content of 0.03 nmolcmÀ2. Pd was barely
observable in the XPS spectra of the used surface (Pd/N
ratio of 0.02) and XRR also denoted a decrease in the con-
tent in electrons of the layers after the first run of catalysis
(870 to 669 electrons nmÀ2). XRR data for more than two
runs indicated probable contamination of the layer, prevent-
ing an accurate evaluation of the amount of remaining Pd.
Nevertheless, the combination of XRR, XPS, and ICP-MS
clearly indicates progressive leaching of Pd from the surface,
which is certainly responsible for the observed drop in reac-
tivity.
Keywords: fluorogenic reactions · heterogeneous catalysis ·
monolayers · palladium · silicon · surface chemistry
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Chem. Eur. J. 2012, 18, 788 – 792