Dissociation and hierarchical assembly of chiral esters on metallic surfaces

Article abstract of DOI:10.1039/c3cc40805a

The interaction of a de novo synthesised ester with single crystal metal surfaces has been investigated as a model system for the heterogeneous hydrogenolysis of esters. Scanning tunnelling microscopy measurements show dissociative adsorption at room temp

Full text of DOI:10.1039/c3cc40805a

Chemical Communications
www.rsc.org/chemcomm
Volume 49 | Number 58 | 25 July 2013 | Pages 6459–6566
ISSN 1359-7345
COMMUNICATION
Giovanni Costantini et al.
Dissociation and hierarchical assembly of chiral esters on metallic surfaces
1359-7345(2013)49:58;1-U

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Dissociation and hierarchical assembly of chiral esters
on metallic surfaces†
Cite this: Chem. Commun., 2013,
49, 6477
Ben Moreton, Zhijia Fang, Martin Wills and Giovanni Costantini*
Received 30th January 2013,
Accepted 12th April 2013
DOI: 10.1039/c3cc40805a
www.rsc.org/chemcomm
The interaction of a de novo synthesised ester with single crystal metal clusters13 indeed showed dissociative adsorption along the RO–C(O)R
surfaces has been investigated as a model system for the heterogeneous bond to generate the RO(Cu) and RC(Cu)O species which then
hydrogenolysis of esters. Scanning tunnelling microscopy measurements undergo hydrogenation to generate the corresponding alcohols.13
show dissociative adsorption at room temperature on Cu(110) but no
Metallic single crystal surfaces are the ideal model systems to
significant reaction on Au(111). The dissociative pathway has been experimentally investigate heterogeneous catalytic reactions. To the
identified by comparing with possible fragment species, demonstrating best of our knowledge, only a few studies have dealt with the deposition
that the ester cleavage occurs along the RCH(CH3)–OC(O)R bond.
of esters on model surfaces. Alkyl and phenyl esters adsorbed at the
solid–liquid interface onto Au(111) did not show any type of dissocia-
Scanning tunnelling microscopy (STM) has developed into a useful tion.14 Similarly, methylacetoacetate deposited in ultra high vacuum
tool for molecular characterisation over recent years. Specifically, (UHV) onto Ni(111)15 and Au(111)16 remained intact. Also methyl
there have been many instances where the chirality of molecules acetate remained stable on Cu(110) unless the substrate was partially
has been inferred from molecular packing1–3 or has been imaged covered by oxygen where it was determined to react with oxygen atoms
unambiguously using STM.4–6 However STM can also be used to to generate methoxy and acetate fragments.17 Only more recently, the
view the starting materials and products of reactions on surfaces7 dissociation of ester protected amines into alkenes and acids has been
and even to initiate them.8 Many of the reactions occurring at solid suggested as one of the steps in a complex reaction mechanism for the
surfaces involve dissociation of the deposited molecular species. creation of surface confined polymers.18
Here we focus on the adsorption and dissociation of a chiral ester
on reactive and unreactive metallic substrates.
In this communication we report the UHV deposition and STM
analysis of a de novo synthesised ester onto both Cu(110) and
Ester cleavage is particularly important for a wide range of Au(111) substrates to directly image the products of dissociative
synthetic reactions, ranging from multitonne hydrolysis of fatty adsorption thereby allowing us to elucidate the reaction pathway.
acids to form soap and detergents, to small scale applications in The two surfaces were chosen as exemplary systems for reactive and
the pharmaceutical and specialist chemical sectors.9 Depending on unreactive substrates, respectively. The (S)-1-(20-naphthyl)ethyl-(4-
the reaction conditions used, the products can alter. For example, in phenylbenzoate) molecule (NEP, Scheme 1) was selected as a model
pyrolysis the decomposition of esters with a b-hydrogen generates system for investigating ester cleavage. NEP contains two structurally
the carboxylic acid and complementary alkene10 while in solution distinct extended aromatic ring systems on either side of the ester,
esters are hydrolysed to the corresponding carboxylic acids and each of which has the potential to bind to a surface. The molecule is
alcohols by using a base. The introduction of heterogeneous catalytic stable during thermal sublimation, the deposition technique used in
mediators such as Rh has been shown to enable hydrogenolysis of this work, as shown using mass spectrometry. Full synthetic and
esters via cleavage of the R–OC(O) R bond, producing the acid and sample preparation details are given in the ESI.‡
an alkane.11 All of these methods generate an acid along with a
Upon adsorption of NEP at room temperature on Cu(110),
separate fragment. However, it has been reported that if Cu is used oblong features appear which are oriented along the [001] and
as catalyst, esters are reduced to the two corresponding primary
alcohols.12 Theoretical modelling performed on heterogeneous Cu
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry,
CV4 7AL, UK. E-mail: g.costantini@warwick.ac.uk; Tel: +44 (0)24 765 24934
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed
issue.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 (S)-1-(20-naphthyl)ethyl-(4-phenylbenzoate) (NEP).
c3cc40805a
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6477--6479 6477

View Article Online
Communication
ChemComm
above as the carboxylate and thus infer that the molecule has
cleaved along the RCH(CH3)–OC(O)R bond.
The main difference between the dissociated NEP and the
biphenyl carboxylic acid sample is the presence of many monomeric
units in the former. Most of these fragments have the shorter of the two
characteristic lengths described above, are asymmetric and a detailed
inspection reveals that they appear in two mirror symmetric forms
(Fig. S5, ESI‡). While the prochiral 2-vinylnaphthalene species gener-
ated by the cleavage of NEP at the RCH(CH3)–OC(O)R bond can explain
this appearance, a cleavage of the RO–C(O)R bond would produce a
fully chiral 1-(2-naphthyl)ethanoyl species (ESI‡) which cannot take two
mirror symmetric conformations. This is further proof that NEP
actually dissociates along the RCH(CH3)–OC(O)R bond.
To further investigate this dissociative adsorption, the effect
of thermal activation on the reaction was studied by reducing
the substrate temperature. Fig. 1(c) shows the structures observed by
depositing NEP on Cu(110) held at ꢁ130 1C which are different from
those observed at room temperature: only 20% of the molecules have
fragmented, while the remaining 80% are still intact. These latter
appear as two lobes, one larger than the other, linked by a bright
protrusion. The lobes can be assigned to the biphenyl and naphtha-
lene rings (large and small lobes, respectively) while the protrusion
(Fig. 1(e)) is identified as the methyl group oriented normal to the
surface. Two main conformations are observed for the intact mole-
cules; 45% of them have an L shape where the lobes are 901 to each
Fig. 1 STM images of NEP deposited on Cu(110). (a) and (b) Room temperature
deposition, the observed features are the result of adsorption-induced fragmen-
tation. (c) and (d) Deposition at ꢁ130 1C, corresponding to intact molecules. The
two main conformations are circled in (c). (e) Linescan over the blue line in (c)
showing the methyl protrusions. All images were acquired at ꢁ196 1C.
%
[110] crystallographic directions of the substrate, Fig. 1(a) and other (highlighted by blue circle in Fig. 1(c)), while in 35% of the cases
(b). These features have two distinct lengths of 11.5 ꢀ 0.4 Å and the lobes are separated by 1301 (yellow circle in Fig. 1(c)). This
12.6 ꢀ 0.6 Å which are significantly smaller than the theoretical difference arises because of a rotation of 1801 around the naphtha-
length of NEP, 18 Å. This reduced length is a first indication lene–CCH3O bond (Fig. S7, ESI‡) and is a further indication that the
that the NEP molecules have adsorbed dissociatively. Cu(110) is molecules are indeed intact. Semi-empirical AM1 calculations show
known to be a very reactive substrate because of the high that in the gas phase these two configurations have identical energies
density of active sites exposed by its atomic-scale corrugation. (ESI‡). Thus, the different observed frequency of the two conforma-
For example, it has been well documented that the room tions must be due to the interaction with the substrate. The remain-
temperature adsorption of benzene-carboxylic acids onto ing 20% of the intact molecules have distinct conformations from the
Cu(110) can cause their deprotonation resulting in surface- main two. Fig. 1(d) shows a zoom of the single molecules where,
stabilised negatively charged carboxylate species.19–22
A similar species, biphenyl carboxylate, could also be one of attribute this imaging mode to a different tip state (ESI‡).23
the possible fragments of NEP if its cleavage occurred at the Finally, in order to investigate the effect of the substrate
however, the methyl groups are not imaged as protrusions. We
RCH(CH3)–OC(O)R bond. The cationic species resulting from this chemistry on the fragmentation of NEP, similar experiments were also
fragmentation is actually observed in the positive mode electrospray performed on the far less reactive Au(111) surface. Room temperature
mass spectrum of NEP (Fig. S1, ESI‡). This secondary cation is deposition on Au(111) resulted in the great majority of molecules
particularly stable in the case of NEP because it is situated next to adsorbing intact. In fact the NEP molecules arrange into chiral
the aromatic naphthalene group. If adsorbed on a metallic surface, hierarchical supramolecular architectures, Fig. 2(a), where the indivi-
the cation is however expected to deprotonate and transform to the dual molecules are imaged in the same manner as on the low
corresponding 2-vinylnaphthalene alkene (see ESI‡). In order to temperature Cu(110). The first level of the hierarchical order is formed
confirm the nature of the fragments, a comparison was made with by H-bonding of individual molecules to organise their smaller lobes
the biphenyl carboxylic acid molecule which is expected to deproto- into a triangle, while the longer lobes point perpendicular to its sides.
nate to the carboxylate upon adsorption on Cu(110).19 The main This arrangement is highlighted by a blue outline in Fig. 2(b). Further
assembly motifs observed for both dissociated NEP and biphenyl packing of the stars into a second level of hierarchy generates larger
carboxylic acid are compared in the ESI‡ (Fig. S3). All of the supramolecular domains, typically extending over 15–20 nm. The
structures observed in the carboxylic acid sample are also present arrangement of the stars always follows the same rotation, indicating
in the dissociated NEP and are explained by ionic H-bonding of the that the chirality of the NEP molecule is propagated throughout the
COOꢁ to the aromatic hydrogens. Although a similar H-bonding hierarchical assembly. Annealing of the sample does not generate
motif could in theory be proposed also for the radical carbonyl larger domains and, above 100 1C, induces desorption of the majority
fragment generated by cleavage of the RO–C(O)R bond, this would of the molecules. The few remaining structures are supramolecular
not fully explain the observed assemblies (Fig. S4, ESI‡). As a arrangements that maintain a triangular symmetry but also include
consequence, we identify the longer of the two fragments described molecular fragments, Fig. 2(c). Three fragments orient in a chiral
c
6478 Chem. Commun., 2013, 49, 6477--6479
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
Cu(110) but that the proton released from the transformation of the
cation to the alkene, instead of recombining and desorbing as
molecular hydrogen, is transferred via the substrate to the nearby
carboxylate fragment, turning it into the acid species (ESI‡). A
concerted b-H elimination pathway could also occur, although this
is expected to be less likely because of the molecular conformation
of NEP on the surface.
Our measurements clearly show that on both surfaces the
ester dissociates along the RCH(CH3)–OC(O)R bond. This is
different from what was reported for heterogeneous Cu catalysis11
and from the dissociation pathway suggested by calculations12 but
agrees with the suggested pathway for Boc-removal proposed in
ref. 18. Possible reasons for this could be the strength of
the interaction of the carboxylate/carboxylic moieties with the
Cu(110)/Au(111) surfaces, which are different from the Cu
clusters used in the calculation, or the stability of the specific
NEP cationic fragment after dissociation.
In conclusion, we have shown that the ester NEP dissociates
at room temperature on Cu(110) and at high temperature on
Au(111) but remains largely intact on low temperature Cu(110)
and room temperature Au(111), where it generates a chiral
hierarchical supramolecular structure. The products of dissocia-
tion have been clearly identified by comparison with biphenyl
carboxylic acid. Based on this, we have demonstrated that on
both surfaces the ester cleavage occurs along the RCH(CH3)–
OC(O)R bond. Moreover, we propose that an effective proton
transfer takes place on Au(111) to generate the acid and
complementary alkenes after the initial bond cleavage.
G.C. gratefully acknowledges financial support from EPSRC
through grant EP/G044864/1, The Royal Society through grant
no. RG100917 and the Warwick-Santander Fund. Some of the
equipment used in this research was obtained through Birmingham
Science City with support from Advantage West Midlands.
Fig. 2 STM image of NEP deposited onto Au(111). (a) and (b) Room tempera-
ture deposition. (b) Zoom of the triangular structures in (a) with the overlaid
molecular model of the packing. The blue outline highlights a single six-pronged
star. (c) and (d) Effect of annealing to 100 1C; the coverage is reduced and the
structures are composed of both whole molecules and molecular fragments. (d)
Zoom of an individual triangular cluster from (c) with the proposed model inset.
All images were acquired at ꢁ196 1C.
manner to form a three-pronged star which also includes three intact
molecules arranged between the prongs, Fig. 2(d). All of these
structures display the same chirality and are aligned with the
fragments along the [101] directions of the surface. As a consequence,
only two 601 mutually rotated orientations are possible. A few features
involving just the fragment species are also observed, such as two
three-pronged stars joined together or a six-pronged chiral propeller
displaying both R- and S-organisational chiralities. We note
that all of these structures can also be found in small percen-
tages on the room temperature high coverage sample, indicat-
ing that Au(111) still has residual catalytic activity although
significantly lower than that of Cu(110).
Also for Au(111) the nature of the fragments was investigated
by a comparative room temperature deposition of the biphenyl
carboxylic acid molecule. The corresponding STM measurements
show assemblies similar to the structures formed by the NEP
fragments as, for example, the three pronged stars (Fig. S10, ESI‡).
The fact that for both Cu(110) and Au(111) the STM images of
the NEP fragments appear very similar to the images obtained by
depositing the biphenyl carboxylic acid allows us to delineate a
hypothesis for the reaction mechanism. In particular, on room
temperature Cu(110) the acid is stable only in its deprotonated
Notes and references
1 S. Haq, et al., Nat. Chem., 2009, 1, 409–414.
2 M. Lingenfelder, et al., Angew. Chem., Int. Ed., 2007, 46, 4492–4495.
3 K.-H. Ernst, in Supramolecular Chirality, ed. M. CregoCalama and
D. N. Reinboudt, 2006, vol. 265, pp. 209–252.
4 V. Demers-Carpentier, et al., Science, 2011, 334, 776–780.
5 F. Masini, et al., J. Am. Chem. Soc., 2011, 133, 13910–13913.
6 S. De Feyter, et al., Langmuir, 1999, 15, 2817–2822.
7 G. Franc and A. Gourdon, Phys. Chem. Chem. Phys., 2011, 13,
14283–14292.
8 S. W. Hla, et al., Phys. Rev. Lett., 2000, 85, 2777–2780.
9 J. S. Carey, et al., Org. Biomol. Chem., 2006, 4, 2337–2347.
10 A. Maccoll, in Adv. Phys. Org. Chem., ed. V. Gold, Academic Press,
1965, vol. 3, pp. 91–122.
11 M. Zdrazil and M. Kraus, Collect. Czech. Chem. Commun., 1974, 39,
3515–3521.
12 J. W. Evans, et al., J. Catal., 1984, 88, 203–213.
13 M. A. N. Santiago, et al., J. Catal., 2000, 193, 16–28.
14 K. Tahara, et al., CrystEngComm, 2011, 13, 5551–5558.
form,19–21 implying by comparison that the ester is being cleaved at 15 T. E. Jones and C. J. Baddeley, Surf. Sci., 2002, 519, 237–249.
16 A. G. Trant and C. J. Baddeley, Langmuir, 2011, 27, 1788–1795.
17 S. L. Silva, et al., Surf. Sci., 2000, 452, 79–94.
18 S. Boz, et al., Angew. Chem., Int. Ed., 2009, 48, 3179–3183.
the RCH(CH3)–OC(O)R bond to form the carboxylate and the
cation, the latter most probably further deprotonating to an alkene.
On the other hand, the biphenyl carboxylic acid molecule is not 19 B. G. Frederick, et al., Surf. Sci., 1993, 292, 33–46.
20 D. S. Martin, et al., Phys. Rev. B, 2002, 66, 155427.
21 T. Classen, et al., J. Phys. Chem. A, 2007, 111, 12589–12603.
22 Y. Wang, et al., Chem. Commun., 2012, 48, 534–536.
expected to deprotonate on Au(111) which means that the fragment
must also be in its acidic form on this surface. We therefore
tentatively suggest that on Au(111) NEP initially fragments as on 23 M. M. Knudsen, et al., J. Am. Chem. Soc., 2011, 133, 4896–4905.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6477--6479 6479