Deprotection, tethering, and activation of a catalytically active metalloporphyrin to a chemically active metal surface: [SAc]4P- Mn(III)Cl on Ag(100)

Article abstract of DOI:10.1021/ja8076389

The adsorption and subsequent thermal chemistry of the acetyl-protected manganese porphyrin, [SAc]₄P-Mn(III)Cl on Ag(100) have been studied by high resolution XPS and temperature-programmed desorption. The deprotection event, leading to formati

Full text of DOI:10.1021/ja8076389

Published on Web 01/20/2009
Deprotection, Tethering, and Activation of a Catalytically
Active Metalloporphyrin to a Chemically Active Metal Surface:
[SAc]4P-Mn(III)Cl on Ag(100)
Mark Turner, Owain P. H. Vaughan, Georgios Kyriakou, David J. Watson,
Lukas J. Scherer, Greg J. E. Davidson, Jeremy K. M. Sanders, and
Richard M. Lambert*
Department of Chemistry, UniVersity of Cambridge, Cambridge CB2 1EW, United Kingdom
Received September 26, 2008; E-mail: rml1@cam.ac.uk
Abstract: The adsorption and subsequent thermal chemistry of the acetyl-protected manganese porphyrin,
[SAc]4P-Mn(III)Cl on Ag(100) have been studied by high resolution XPS and temperature-programmed
desorption. The deprotection event, leading to formation of the covalently bound thioporphyrin, has been
characterized and the conditions necessary for removal of the axial chlorine ligand have been determined,
thus establishing a methodology for creating tethered activated species that could serve as catalytic sites
for delicate oxidation reactions. Surface-mediated acetyl deprotection occurs at 298 K, at which temperature
porphyrin diffusion is limited. At temperatures above ∼425 K porphyrin desorption, diffusion and deprotection
occur and at >470 K the axial chlorine is removed.
Introduction
complementary chemistry of the metal surface, for example to
produce systems capable of carrying out delicate selective
The bottom-up fabrication of systems consisting of functional
molecules tethered to solid surfaces is an important research
area of relevance to a wide range of applications including
electrochemistry, molecular electronics, and catalysis.1 Robust,
reproducible molecule-surface tethering is important and is often
achieved by means of thiol linkages. However, the reactivity
of thiol groups is such that they need to be protected during
synthetic procedures and then deprotected to enable covalent
linkage to the solid surface. Recent interest in the electrochemi-
cal and physical properties of metalloporphyrins has stimulated
work on the covalent attachment of these macrocycles to
electroactive surfaces, an important example being the synthesis
of thiol-functionalized porphyrins,2-10 led by the work of
Lindsey et al.3 Our interest lies in a hitherto neglected field:
the attachment of catalytically active metalloporphyrins to metal
surfaces to create hybrid catalytic systems in which the
characteristic chemistry of the porphyrin is harnessed to the
oxidations. We choose silver because at ambient temperature it
can dissociatively adsorb dioxyen and π-adsorb alkenes, thus
preparing the reactants in the desired state.
We have previously made a porphyrin-functionalized silver
surface that exhibits ligand binding, unbinding, and displacement
reactions characteristic of the free metalloporphyrin11 and have
also used well-chosen ligands to dramatically alter their dynam-
ics.12 Such weakly bound systems are unlikely to be sufficiently
robust for catalytic applications, however, especially in the
presence of a solvent where resistance to leaching is an essential
attribute.
Very recently, we reported the first direct observation by
scanning tunneling microscopy (STM) of spontaneous surface-
mediated deprotection of a free base thiol-terminated porphyrin
([SAc]4P-H2) on Ag(100):13 such tethered porphyrins possess
no inherent catalytic properties, of course. Here we extend this
work into the domain of chemically active metalloporphyrins
that are known to catalyze oxygen transfer reactions in
(1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides,
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Chem. Soc. 1995, 117, 9529.
Figure 1. Molecular structure and 3D space-filling model of
[SAc]4P-Mn(III)Cl.
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[SAc]4P-Mn(III)Cl on Ag(100)
A R T I C L E S
Figure 2. S 2p region of XP spectra acquired (a) immediately following porphyrin deposition and (b) after subsequent annealing at 473 K. As a guide, the
XP spectra are overlaid with a representative STM image of an equivalent surface state for the free base porphyrin, [SAc]4P-H2, taken from ref 13 (left
image: 900 Å × 900 Å, Vtip ) 1.5 V, I ) 0.1 nA; right image: 400 Å × 400 Å, Vtip ) 0.75 V, I ) 0.05 nA).
Figure 3. (a) S 2p region of XP spectra taken with increasing [SAc]4P-Mn(III)Cl porphyrin dose time. In each case the surface was annealed at 473 K for
15 min prior to recording the spectrum. (b) Integrated S 2p intensity for each curve, plotted with respect to porphyrin dose time. (c) Associated Ag 3d
integrated intensities.
solution.14-21 As explained above, this is the next step toward
the creation of hybrid heterogeneous selective oxidation cata-
lysts. Our aim was to deposit the acetyl-protected porphyrin
[SAc]4P-Mn(III)Cl (Figure 1) on Ag(100), deprotect it by
removal of the acetyl groups thus enabling covalent tethering
to the surface by Ag-S bonds, and finally activate it by removal
of the axial chlorine ligand. The actual sequence of events was
followed under well-defined conditions by means of synchrotron
high-resolution X-ray photoelectron spectroscopy (XPS) and
temperature-programmed desorption (TPD). In regard to their
homogeneous chemistry, manganese porphyrins are well-known
for their effectiveness as selective oxidation catalysts when used
in conjunction with a single-oxygen donor.14-21 In the present
case, the silver surface bearing oxygen adatoms is envisaged
as the entity that will act the single-oxygen donor.
Experimental Methods
The free base 5,10,15,20-tetrakis[3-(S-acetylthiomethyl)phe-
nyl]porphyrin was synthesized using the method described by
Lindsay et al.3 Metalation of the free-base porphyrin was achieved
in a mixture of acetic acid:acetic anhydride ) 4:1 at 110 °C using
MnCl2 ·4H2O as the manganese source. After completion of the
reaction, the remaining inorganic salts were filtered, leaving the
pure [SAc]4P-Mn(III)Cl porphyrin.
High resolution XPS was carried out on the SuperESCA beamline
at the ELETTRA synchrotron radiation source in Trieste, Italy.
Spectra were collected using a single-pass 32-channel concentric
hemispherical electron analyzer, and the angle between the entrance
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A R T I C L E S
Turner et al.
Figure 4. S 2p region of the XP spectrum acquired after initial deposition
and following 10 h left at room temperature. The protected S contribution
is represented by red hatching; deprotected S is represented by blue hatching.
lens of the analyzer and the incoming photon beam was 70° in the
horizontal plane. Experiments were performed with a photon
incidence angle of 10° with respect to the surface. The Ag(100)
crystal was attached to a motorized manipulator via a tantalum
backplate fitted with a T1T2 thermocouple and could be heated
resistively to 900 K or cooled to 77 K. Repeated cycles of Ar+
(99.999% Messer) sputtering followed by annealing at 800 K were
carried out until a clean, atomically flat surface was obtained, as
monitored by XPS and LEED.
Temperature-programmed desorption experiments were con-
ducted in Cambridge in an ultrahigh vacuum chamber operated at
a base pressure of 1 × 10-10 torr. The sample was exposed to gases
by backfilling the chamber which was equipped with an Omicron
3 grid retarding field analyzer for LEED/AES analysis and a VG
300 quadrupole mass spectrometer whose ionizer was positioned
5 mm from the front face of the sample. The Ag(100) single crystal
could be cooled to 100 K and heated to 1400 K, monitored by a
T1T2 thermocouple attached directly to the sample. The sample
was cleaned by repeated cycles of Ar+ (99.999% Messer) sputtering
followed by annealing at 700 K until a clean atomically flat surface
was obtained, as observed by LEED and AES.
Figure 5. (a) C 1s region of the temperature-programmed XP spectrum
acquired while applying a 5 K s-1 temperature ramp from 298 to 523 K.
(b) C 1s integrated intensity of each curve plotted with respect to
temperature.
Porphyrins were deposited onto the Ag surface, both in Cam-
bridge and in Trieste, by means of a resistively heated collimated
evaporation source fitted with a T1T2 thermocouple. The (very
small) amounts of porphyrin used were injected into the sublimation
source as solutions in dichloromethane which were evaporated to
dryness before mounting in the vacuum chamber.
Results and Discussion
Figure 2a shows the S 2p spectrum acquired following initial
deposition of the porphyrin with the Ag(100) sample at 300 K.
Three peaks are clearly evident which are due to two distinct,
overlapping S 2p1/2,3/2 doublets. The emission at higher binding
energy of 163.7 eV (red hatching) is due to thiol groups that
are still acetyl protected. The component at lower binding
energy, centered at 162.3 eV (blue hatching), may be confidently
assigned22 to deprotected thiol groups covalently bonded to the
silver surface, after cleavage of the acetyl group.
Figure 6. Temperature-programmed desorption data acquired after dosing
[SAc]4P-Mn(III)Cl onto clean Ag(100). Curves correspond to: m/z ) 43
(CH3CO+), m/z ) 35 (Cl+), m/z ) 32 (S+).
that used for dispersing and deprotecting the closely related free
base porphyrin [SAc]4P-H2 where we were able to follow the
surface processes by means of STM.13 In that case, verification
of the expected ratio of cleaved acetyl groups to deprotected
porphyrin molecules was possible: in the present case the C 1s
Annealing at 473 K for 15 min resulted in the XP spectrum
shown in Figure 2b. Note that this procedure is the same as
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[SAc]4P-Mn(III)Cl on Ag(100)
A R T I C L E S
Figure 7. Cl 2p XP spectrum acquired after initial deposition of [SAc]4P-Mn(III)Cl (left) and after subsequent annealing at 473 K for 15 min (right).
emission are consistent with the low number density of Cl and
Mn atoms on the surface even at saturation coverage, even
though both were examined at the optimum photon energies
that maximized the photoionozation cross sections: Cl 2p ) 3
Mb, Mn 2p ) 1 Mb.23)
It is apparent from Figure 2 that at room temperature a small
degree of thiol deprotection occurs immediately upon deposition.
This implies that cleavage of acetyl groups is surface-mediated
and that the process is limited by the inability of most porphyrin
molecules to access the surface as suggested by the inset to
Figure 2a (i.e., the relevant STM image): the majority are present
in large aggregates, and only those at the periphery can undergo
deprotection. Elevated temperatures increase porphyrin diffusion
and deprotection (Figure 2), although even at room temperature,
given sufficient time, a significant degree of deprotection occurs.
Figure 4 illustrates a S 2p XP spectrum acquired 10 h after
initial S deposition; the situation is intermediate between the
two extreme cases of total protection/deprotection shown in
Figure 2.
Figure 5a shows a series of temperature-programmed C
Figure 8. S 2p emission as a function of thermal treatment: spectra acquired
after annealing the porphyrin-covered surface to the indicated temperature
for 5 min.
1s XP spectra acquired over the interval 298 K to 523 K
with a ramp rate of 5 K s-1, after deposition of [SAc]4P
-Mn(III)Cl. Upon porphyrin deposition, two C 1s signals
emission does not distinguish between the acetyl carbon atoms
and macrocycle carbon atoms. For the purpose of illustration,
the corresponding images taken from ref 13 are shown as insets
to Figure 2. It is apparent from Figure 2b that 473 K annealing
leads to essentially complete deprotection of remaining por-
phyrin. The presence of a single S 2p signal clearly indicates
that all thiol groups are bonded to the silver surface.
(11) Williams, F. J.; Vaughan, O. P. H.; Knox, K. J.; Bampos, N.; Lambert,
R. M. Chem. Commun. 2004, 1688.
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Angew. Chem., Int. Ed. 2006, 45, 3779.
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1993, 115, 7985.
Figure 3a shows the uptake of [SAc]4P-Mn(III)Cl as
monitored by the S 2p emission; the right panel shows a plot
of corresponding integrated intensities of the S 2p and Ag 3d
emission as a function of dosing time. These data were acquired
by depositing the porphyrin at room temperature followed by
annealing at 473 K for 15 min to disperse the initially formed
2D islands and desorb any multilayer material, a process that
occurred at ∼425 K (see below): this deposition method
provided a convenient way of preparing any given coverage in
the submonolayer to monolayer regime. The procedure was then
repeated in order to build up coverage from the submonolayer
regime to that of a fully saturated covalently bonded monolayer.
(The weak Cl 2p emission and the undetectability of Mn 2p
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A R T I C L E S
Turner et al.
were observed at 285.6 and 288.1 eV. Upon raising the
temperature, the 288.1 eV component attenuated rapidly,
disappearing by 523 K, at which point only the lower binding
energy component remained, slightly downshifted to 285.4
eV. These two components are ascribed, respectively, to
porphyrin molecules in contact with the Ag surface (285.6
eV) and molecules in the second or higher layers (288.1 eV),
the lower binding energy of the former reflecting greater final
state screening of the C 1s core hole by the Ag valence
electrons.24
Figure 5b, showing the total integrated C 1s intensity as a
function of temperature, is consistent with this interpretation.
The initial slow decrease in the C 1s integrated intensity is
principally due to evaporation of the more weakly held
“multilayer” molecules, followed by a much faster decrease
that sets in at 410 K due to desorption of molecules in the
contact layer. The limiting C 1s intensity at 523 K decreased
by a further ∼10% upon heating to 573 K.
The Cl+ desorption peak at ∼515 K corresponds to loss of
the manganese-bound chlorine ligand, and in this connection
the XPS results illustrated in Figure 7 are revealing. Examination
of the Cl 2p emission before and after annealing the porphyrin
layer at 473 K (onset of chlorine loss in Figure 6) for 15 min
shows complete removal of Cl from the surface. It is well-known
that Cl chemisorbs strongly on silver surfaces forming AgCl25
which acts to assist porphyrin dechlorination during the
relatively mild thermal treatment. This process both deprotects
and immobilizes the porphyrin and leaves the manganese metal
center in a low-coordination state, thus preparing it as a
catalytically active site.
At temperatures above ∼560 K the deprotected porphyrin
molecules decompose with desorption of sulfur (Figures 6, 8)
and formation of a carbonaceous residue (Figure 5a,b). The S
2p intensity that persists after heating to 673 K (Figure 8) is
consistent with Figure 6 from which it is apparent that S
desorption is incomplete at this temperature.
Representative TPD measurements carried out after depo-
sition of [SAc]4P-Mn(III)Cl on Ag(100) are shown in Figure
6 for m/z ) 43 (CH3CO+), m/z ) 35 (Cl+), m/z ) 32 (S+), the
three data sets being acquired simultaneously; m/z values >100
were beyond the range of the quadrupole mass spectrometer.
The maximum at 425 K in the m/z ) 43 profile is assigned to
the CH3CO+ fragment ion due to protected porphyrin molecules
desorbing from the contact layer (m/z ) 43 was an intense signal
in the mass spectrum of the porhyrin vapor from the evaporation
source). This is in good agreement with the XPS results (Figure
5b) which exhibit a steep decline in C 1s intensity over the
same temperature range. The TPD result also implies that upon
raising the temperature some of the acetyl-protected molecules
desorb intact, whereas others undergo deprotection and become
strongly bound to the surface via S-Ag linkages. Correspond-
ingly, the coincident feature at 425 K in the m/z ) 35 spectrum
is assigned to the Cl+ fragment ion from intact desorbing
porphyrin molecules.
Conclusions
The acetyl-protected manganese porphyrin, [SAc]4P-Mn
(III)Cl may be deposited on Ag(100) and subsequently depro-
tected, leading to stable, covalently bound thioporphyrin
molecules. These in turn may be dechlorinated, resulting in
formation of stable, surface-tethered low-coordinate Mn centers,
thus establishing a methodology for creating thio-tethered
activated species that could serve as catalytic sites for delicate
oxidation reaction. These species appear to be stable to over
500 K which is well above the temperature range of the intended
application.
Acknowledgment. M.T. and O.P.H.V. acknowledge financial
support from Johnson Matthey plc and King’s College, Cambridge,
respectively. G.J.E.D. acknowledges financial support from NSERC
Canada. L.J.S. acknowledges financial support from the Swiss
National Foundation and the Novartis Foundation.
JA8076389
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