The Scope of Direct Alkylation of Gold Surface with Solutions of C1-C4 n-Alkylstannanes

Article abstract of DOI:10.1021/jacs.5b07672

Treatment of cleaned gold surfaces with dilute tetrahydrofuran or chloroform solutions of tetraalkylstannanes (alkyl = methyl, ethyl, n-propyl, n-butyl) or di-n-butylmethylstannyl tosylate under ambient conditions causes a self-limited growth of disordered monolayers consisting of alkyls and tin oxide. Extensive use of deuterium labeling showed that the alkyls originate from the stannane and not from ambient impurities, and that trialkylstannyl groups are absent in the monolayers, contrary to previous proposals. Methyl groups attached to the Sn atom are not transferred to the surface. Ethyl groups are transferred slowly, and propyl and butyl rapidly. In all cases, tin oxide is codeposited in submonolayer amounts. The monolayers were characterized by ellipsometry, contact angle goniometry, polarization modulated IR reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy with ferrocyanide/ferricyanide, which revealed a very low charge-transfer resistance. The thermal stability of the monolayers and their resistance to solvents are comparable with those of an n-octadecanethiol monolayer. A preliminary examination of the kinetics of monolayer deposition from a THF solution of tetra-n-butylstannane revealed an approximately half-order dependence on the bulk solution concentration of the stannane, hinting that more than one alkyl can be transferred from a single stannane molecule. A detailed structure of the attachment of the alkyl groups is not known, and it is proposed that it involves direct single or multiple bonding of one or more C atoms to one or more Au atoms.

Full text of DOI:10.1021/jacs.5b07672

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Article
The Scope of Direct Alkylation of Gold Surface
1
4
with Solutions of C - C n-Alkylstannanes
Eva Kaletova, Anna Kohutova, Jan Hajduch, Ji#í Kaleta, Zdenek Bastl,
Ivan Stibor, Lubomir Pospisil, Thomas F. Magnera, and Josef Michl
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07672 • Publication Date (Web): 01 Sep 2015
Downloaded from http://pubs.acs.org on September 2, 2015
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The Scope of Direct Alkylation of Gold Surface with Solutions of C - C n-Alkylstannanes
1 4
a a a a b b
Eva Kaletová, Anna Kohutová, Jan Hajduch, Jiøí Kaleta, Zdenìk Bastl, Lubomír Pospíšil, Ivan
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a
c
a,c
Stibor, Thomas F. Magnera, and Josef Michl *
a
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
1
6610 Prague 6, Czech Republic
b
J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 18223
Prague 8, Czech Republic
c
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado
8
0309-0215, U.S.A.
Abstract
Treatment of cleaned gold surfaces with dilute tetrahydrofuran or chloroform solutions of
tetraalkylstannanes (alkyl = methyl, ethyl, n-propyl, n-butyl) or di-n-butylmethylstannyl tosylate
under ambient conditions causesa self-limited growth of disorderedmonolayers consisting of alkyls
and tin oxide. Extensive useofdeuteriumlabelingshowedthatthealkyls originate from the stannane
and not from ambient impurities, and that trialkylstannyl groups are absent in the monolayers,
contrary to previous proposals. Methyl groups attached to the Sn atom are not transferred to the
surface. Ethyl groups are transferred slowly, and propyl and butyl rapidly. In all cases, tin oxide is
co-deposited in submonolayer amounts. The monolayers were characterized by ellipsometry,
contact angle goniometry, polarization modulated IR reflection absorption spectroscopy, X-ray
photoelectron spectroscopy, and electrochemical impedance spectroscopy with
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ferrocyanide/ferricyanide, which revealed a very low charge-transfer resistance. The thermal
stability of the monolayers and their resistance to solvents are comparable with those of an n-
octadecanethiolmonolayer. Apreliminaryexaminationofthekineticsofmonolayerdeposition from
a THF solution of tetra-n-butylstannane revealed an approximately half-order dependence on the
bulk solution concentration of the stannane, hinting that more than one alkyl can be transferred from
a single stannane molecule. A detailed structure of the attachment of the alkyl groups is not known
and it is proposed that it involves direct single or multiple bonding of one or more C atoms to one
or more Au atoms.
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Introduction
The self-limiting formation of alkyl-containing monolayers on metal surfaces by treatment
1
,2,3,4,5,6
with alkanethiols has been known for over half a century,
and has been of much use in recent
7
decades for various applications in nanoscience and nanotechnology. Such sulfur-mediated
attachment of alkyls and other organic moieties to the surface of gold has many advantages,
especially easy generation in solution under ambient conditions, and a few disadvantages, such as
8
,9,10
11,12
moderate but distinct sensitivity to oxidation
and relatively poor electrical conductivity.
13,14
The much more recently discovered
self-limiting formation of alkyl-containing
monolayers on gold surface by treatment with ambient solutions of alkylstannanes of the types
(
C H ) SnX, (C H ) (CH )SnX, and (C H )(CH ) SnX, where the leaving group X is triflate,
18 37 3 18 37 2 3
18 37
3 2
trifluoroacetate, or tosylate, offers an alternative easy access to alkyl-covered gold surfaces. The
resulting monolayers do not contain the leaving group X and initially we thought that they contain
trialkylstannyl groups attached to the gold surface through Sn-Au bonds. It was puzzling that they
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had the same ellipsometric thickness and other properties regardless of which of the nine stannane
precursors was used. Some of these properties are significantly different from those of alkanethiol-
based monolayers. In particular, the monolayers formed from the stannanes are somewhat more
resistant to oxidation, and above all, are disordered and block an electrode surface onlyveryweakly.
While permeabilitywould be a disadvantage in manyapplications, it could be an advantage in others
that require simultaneous gold surface functionalization and solute access.
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In retrospect, the puzzling observation that all nine stannanes examined producedessentially
identical monolayers of the same ellipsometric thickness would have been most simply rationalized
by postulating that from any one of the reagents one or more long alkyl chains were transferred to
the gold surface under formation of C-Au bonds and the rest of the reagent was lost to solution, but
this was not proposed. It is the conclusion reached presently, except that we have now discovered
that under ambient conditions tin oxide is codeposited on the surface along with the alkyl groups.
1
5
The first step toward such a recognition was taken by another group of authors, who
measured remarkably high single-molecule conductivities starting with alkane chains terminated
with trimethylstannyl groups on each end for attachment to gold electrodes, and proposed that an
alkyl-trimethylstannyl bond wascleavedandboth residuesweredirectlyattachedto thegold surface.
Their experiment did not permit other types of measurement, but theyprovided convincing evidence
forC-Aubond formationbycomparisonwith experimentsthatstartedwith analkyl chainterminated
1
6,17,18
19
on each end with phosphine-protected gold atoms. Additional conductivity
and mechanistic
studies have appeared since and it is now accepted that the attachment to the surface occurs through
C-Aubonds. Theauthorsprovidedno evidencefortheproposedpresenceoftrimethylstannyl groups
on the gold surface and we shall see below that they are indeed absent. Their mechanism does not
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account for the formation of identical monolayers from the nine stannanes investigated earlier, but
the recognition that C-Au bonds were formed was a critical step forward.
Weobtainedindependentevidenceforalkylattachmenttoagold surfacethroughdirectC-Au
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bonding by reaction with a main group organometallic starting with alkylmercury derivatives, with
20
21
eitheralong(n-C H ) and a short (C H ) chain. This proof was provided byX-rayphotoelectron
37
1
8
4
9
spectroscopic measurements of atomic surface concentrations in the initially produced films and,
most convincingly, in films from whose surface mercury was removed, either by electrochemical
anodic stripping or simply by heating. Interestingly, our initial experiments with stannanes were
2
2,23,24
motivated by observations of the adhesion of certain organomercurials to gold
and, more
distantly, the even earlier observations of the adhesion of organoplatinum compounds to platinum
2
5
surfaces.
Attachment of organic groups other than alkyls by direct C-Au bonding is also known and
2
6,27
can be accomplished, for instance, by treatment with aryldiazonium salts,
terminal
2
8,29,30
31
andcarbenes. Single-moleculejunctions obtainedbymetal-carboncouplingwere
acetylenes,
32,33
34
also reported for fullerene
and benzene.
The purpose of the present paper is to explore the scope of the alkylation of gold surface
under ambient conditions with organostannanes carrying short alkyls, methyl to n-butyl. We have
used a series of stannanes 1 - 15 (Table 1) carrying either four alkyls or three alkyls and a tosylate
leaving group on a tin atom, and characterize the properties of the resulting alkyl monolayers. We
usedeuteriationtodifferentiatethealkyls transferredfrom a stannanefrom contaminantsoriginating
in random impurities, whose signals are otherwise difficult to exclude when working in solution in
open air.
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The co-deposition of tin compounds is a disadvantage if purely alkyl coated surfaces are
3
5
desired. As is to be reported in detail elsewhere, it can be avoided if desired by using solutions of
dibutylditosyloxystannane dihydrate, which appear to coat gold surfaces with butyl groups only,
without depositing detectable amounts of tin. We have already noted above that a metal-free
butylated gold surface can also be produced with solutions of n-butylmercury tosylate followed by
subsequentremovalofmercury. However,in viewoftheirtoxicity, organomercurialsareanunlikely
candidate for widespread use. Even tin is a toxic element, but an original attempt to replace it with
a lighter congener failed when solutions of numerous long-chain alkylsilanes were found not to
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deposit monolayers on gold surfaces under ambient conditions.
The direct alkylation of gold surfaces with solutions of organometallics under ambient
conditions is still in its infancy and the present finding that methyl is not transferred and ethyl is
transferredmuch more slowlythan longer alkyls, and that tin oxide is also deposited, onlyrepresents
a first step toward establishing the detailed structure of the attachment to the gold surface and the
mechanism of its formation. It is possible that the terminal carbon atom is attached to one or more
gold atoms of the surface and the alkyl is intact, but it is also conceivable that it has lost one or more
of its hydrogen atoms and is attached to the surface in a more complicated mode, e.g., as an
alkylidene. In the following text, we use the term alkyl in a loose fashion without implying
attachment through a single C-Au bond.
Table 1. Organostannanes 1 - 15
R R R R Sn
1
2
3
4
R
1
R
2
R
3
R
4
1
CD
CD
CD
CD
3
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CD CD
2
CD CD
2
CD CD
2
CD CD
2
3
3
3
3
CD CH CH
3 2
CD CH CH
3 2
CD CH CH
3 2
CD CH CH
3 2
2
2
2
2
CH CH CH
3
2
CH CH CH
3
2
CH CH CH
3
2
CH CH CH
3
2
2
2
2
2
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0
CD CD CD CD
3 2 2
CD CD CD CD CD CD CD CD CD CD CD CD
2 2 3 2 2
2
2
2
2
2
2
2
2
2
2
2
3
2
2
3
2
2
2
CH CH CH CH
3
2
2
CH CH CH CH CH CH CH CH CH CH CH CH
2
2
2
3
2
2
2
3
2
2
2
2
2
3
CH CH CH CH
3 2 2
CH CH CH CH CH CH CH CH
3 2 3 2 2
CH
2
2
3
3
3
3
3
3
3
CH CH CH CH
3 2 2
CH CH CH CH CH CH CH CH
CD
CH
CD
CH
CD
CH
CD
3
2
2
2
2
2
3
2
2
CH CH CH CH
3 2 2
CH CH CH CH
3 2 2
CH
3
3
3
3
3
3
3
1
0
1
2
3
4
5
CH CH CH CH
3 2 2
CH CH CH CH
3 2 2
CD
CH
CD
CH
CD
CH
1
1
1
1
1
CH CH CH CH
3 2 2
CH
3
3
3
3
CH CH CH CH
3
2
2
CD
CH
CD
CD CD CD CD
3
2
2
CD CD CD CD
3
2
2
3
a
CH CH CH CH
3 2 2
CH CH CH CH
3 2 2
OTs
2
a
Ts = p-toleuenesulfonyl.
Thepresentation of the results is organized as follows. We first provideinitial evidencefrom
measurements of the contact angle á and the ellipsometric thickness d that after a long enough time
(several hours), the same film, except for possible deuteriation, is produced from the THF solutions
of all the stannanes carrying at least one n-butyl, 5 - 15, whereas a different film is produced from
the propyl-carrying stannanes 3 and 4. Yet another film is produced much more slowly from the
ethyl-carrying 2. No significant monolayer formation is observed with the methyl-carrying 1.
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Second, we examine the kinetics of the observed self-limited growth of ellipsometric
thickness of the film produced from 6 in a cursory fashion and note that it is of fractional order in
the bulk stannane concentration. This observation calls for a more detailed future study.
Third, we describe the IR spectra of the films, providing further evidence for their identity
and pinpointing which alkyls do and which ones do not transfer to the surface. This definition of the
reaction scope is the main topic of the paper. A detailed examination of the IR spectra is planned
for the future and should include an investigation of the time dependence of their shape during the
deposition, which is especially noticeable for 2, and an investigation of the spectra obtained with
stannanes carrying alkyls partially deuteriated in specific positions.
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Fourth, we investigate the elemental composition of the alkylated surfaces by XPS and
demonstrate the presence of tin oxide on all stannane-treated gold surfaces, even those treated with
1
, which do not carry alkyls.
Fifth, we measure electrode blocking properties of the films by cyclic voltammetry and
electrochemical impedance spectroscopy and determine their charge transfer resistance.
Sixth, we report some observations on the thermal stability of the films.
Results
Growth and Macroscopic Properties of the Monolayers. An initial indication of
monolayer formation was provided by a gradual change of ellipsometric thickness d and contact
angle á toward reproducible final values after keeping a cleaned gold surface inside a dried THF
solution of one of the stannanes 1 - 15 for periods ranging from a few min to 18 h, followed by
thorough rinsing and drying. Chloroform solutions were also used for the stannanes 1, 2, 3, and 12
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and produced monolayers with similar d values and IR spectra as those deposited from THF. We
have not detected any difference between monolayers deposited using different concentrations of a
stannane, as long as sufficient time was provided to reach the long-term limit. Unless specified
otherwise, all results described below refer to monolayers deposited from THF solutions.
The evaluation of the ellipsometric thickness d requires a knowledge of the index of
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refraction. The value 1.47 is ordinarily considered appropriate for alkyl chains, but as we shall see
below, in our case tin oxide is codeposited with the alkyls and would by itself require a value of
37
.0. An additional uncertainty exists in that some or all of the tin might be present in the form of
2
tin monoxide or suboxide. Since we shall only use the d values in a relative sense, the exact value
of the index of refraction is not critical. Arbitrarily, we have adopted the average of the two indices
and have used the value 1.74 in all ellipsometric evaluations except for those films that contain no
alkyls, in which case we used 2.0, and the alkanethiol films used for comparison, in which case we
used 1.47.
In pure solvent, THF or chloroform, d does not increase within the experimental error over
-4 -3
a period of 18 hours. In 1×10 to 1×10 M THF solutions of the methyl-carrying 1 the changes in
á and d upon monolayer deposition were very small and barely outside the experimental error. In
-4
×10 M THF solutions of ethyl-carrying 2, the changes in á and d were very slow. The limiting
1
-4
values were only reached after about 18 hours and then remained stable. In 5×10 M THF solution,
they were reached after 4 - 5 hours. In the case of the n-propyl-carrying 3 and 4, and of the n-butyl-
-4
carrying 5 - 15, in 1×10 M THF solutions, both d and á changed distinctly and rapidly at first and
converged after a few hours to limiting values that then remained stable. These final values were
á = 69° ± 4° and d = 1.8 ± 0.5 Å for the ethyl containing 2, á = 77° ± 4° and d = 2.8 ± 0.3 Å for
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the propyl containing 3 and 4, and á = 90° ± 3° and d = 4.3± 0.8 Å for the butyl containing 5 - 15.
4
4
For CHCl solutions, the final values of d were determined for 1, 2, 3 and 12, and were found to be
3
the same as for THF solutions. Figure 1 shows the final values of the static contact angle á.
Figure 2A displays the overall time dependence of the thickness d obtained with all of the
stannanes. Figure 2B focuses on a selection of six stannanes carrying butyls and methyls and
demonstrates that isotopic substitution in either alkyl has a small or no effect on the growth rate
within thelimits imposedbytheexperimentalaccuracy. Althoughwerecognizethelimited accuracy
of the ellipsometric method for measuring the degree of coverage, we considered it worthwhile to
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-5
attempt a preliminaryglobal fit of the growth curves of 6 for concentrations ranging from 10 to 10
-2
M to a kinetic model, as described next (Figure 3).
Figure 1. The static contact angle á of water on a cleaned Au
substrate (0) and its limiting value on films produced after 5 h from
!4
×10 M THF solutions of the stannanes 1 - 15 and from n-
1
!4
C H SH (16). For the slowly reacting stannane 2, a 5×10 M
18 37
solution was used.
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1
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0
1
2
3
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5
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8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. Time dependence of ellipsometric thickness d of a
-4
monolayer deposited on a cleaned Au substrate from a 10 M
solution in THF. (A) Monolayers of 1 - 15, compared with n-
C H SH (16) and pure solvent (0). Estimated precision: ± 0.3 D
18
37
for 3 and 4 and ± 0.8 D for 5 - 15. (B) Detail: stannanes that differ
by deuteriation.
1
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Figure 3. Global fits for concentration-weighted growth curves for compound 6
done for simple Langmuir kinetics (A, B) and surface-limited Langmuir kinetics (C,
D) with first-order (A, C) and fit-determined order (B, D) of c. The data are
comprised of five separately acquired sets with c : 0.01 M (black), 0.001 M (red),
0
.0005 M (blue), 0.0001 M (magenta) and 0.00001 M (green).
Kinetic Model. A reasonable starting point is to assume that the surface reaction of the
38,39,40
alkylstannanes follows the well-studied behavior of alkanethiols and dialkyl disulfides
and
proceeds through two steps, (i) reversible physisorption of the stannane reagent on the gold surface,
and (ii) an irreversible surface reaction consisting of a transfer of one or more alkyl groups from the
tin atom of the physisorbed stannane to the gold surface. We have taken the è(t,c) = d(t,c)/d value
4
as a crude approximation to the fraction of the surface that has been coated at time t in a THF
solution containing a constant molar concentration c of a stannane.
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5
5
6
We first note that under the reaction conditions the overall process is irreversible in that d
4
is independent of c and once produced, the films are not removed by the pure solvent even after a
long time. Although an alkylstannane can have as many as four different alkyl groups, we are only
dealing with stannanes 1 - 15 carrying up to two different alkyl groups on the tin atom. Recognizing
that most n-alkyl groups can be transferred to the surface but methyl cannot, a representation of the
standard mechanism is:
0
1
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9
0
R Me Sn W R Me Sn*
n 4-n n 4-n
(physisorption)
(1)
R Me Sn* 6 m R-Au + R Me Sn*
n 4-n n-m 4-n
(chemisorption)
(2)
where R is an alkyl that can be transferred from tin to the gold surface, and Me is methyl, which
cannot be transferred. Reaction (2) may consist of several steps and is followed by (or concurrent
with) other transformations of the tin-containing product, such as air oxidation.
We have selected the stannane 6 for a more detailed examination. Fits of the data shown in
Figure 3 with models that describe processes in which the film thickness develops through pure
4
1
38
42
diffusion-limited, diffusion-convection-limited, and rapid reversible physisorption were not
satisfactory, as they either produced an incorrect temporal dependence of è or required
physisorption/desorption rate constants that were very large and clearly not rate-limiting. Simpler
models with fewerfitted parameterswerefound to bethemost statisticallymeaningful. Aregression
analysis showed that as the number of parameters increased the curve followed the data better, but
the fitted parameters became less self-consistent.
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If the surface concentration of available Au sites is (1 - è), and if one assumes that the pre-
equilibrium shown in step (1) is fast, such that the surface concentration of R Me Sn* is
n 4-n
q
proportional to the bulk concentration c, the resulting rate law, dè(t)/dt = kc [1 - è(t)], integrates
10
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q
to è(t) = 1 - exp(-kc t) for simple Langmuir kinetics (q is the order of the surface alkylation reaction
38
in c and equals 1/m). If the attachment rate is affected by prior coverage, islands, etc., k can be
q
further adjusted by a second (1 - è) factor to yield the alternative rate law dè(t)/dt = kc [1 - è(t)]
2
q q
(surface-limited Langmuir model). This integrates to è(t) = c t/(1 + kc t).
We have investigated the fits of the data to both rate laws, treating d for each run as an
4
adjustable parameter and starting with the arbitrarily chosen case q = 1 (Figure 3). We then
optimized the value of q for the best global fit and found the reaction orders q = 0.45 for the simple
and q = 0.46 for the surface-limited Langmuir model (Figure 3 and Table 2). A first-order
dependence on c can be unequivocally ruled out, suggesting that more than one alkyl group is being
transferred from a tin atom (m > 1), and within the uncertainty in the fit parameters the order is 1/2.
It appears that the surface-limited Langmuir model is somewhat superior to the simple Langmuir
model, but it is difficult to decide this with certainty. A decision between these and perhaps other
possibilities will clearly require a dedicated kinetic study using a more accurate method of surface
38
43
coverage assessment than ellipsometry, such as those based on resistivity, reflectivity , SHG
44,45
4
2
or a quartz microbalance. This more complete study will also have to use several alkylstannanes
with different numbers of transferable alkyl groups n. Such a study lies outside the framework of
the present paper, which focuses on establishing the scope of the surface alkylation reaction.
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2
2
2
2
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4
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4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 2. Fitted rate constant and film thickness for gold surface alkylation by 6 in a THF solution.
simple Langmuir kinetics
surface-limited Langmuir kinetics
a
b
-q
-1
k (M C s )
c
a
b
-q
-1
c
a
q
d (D)
4
q
d (D)
4
k (M C s )
398 ± 1010
3.60 ± 0.96
q
0
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9
0
1
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9
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1
2
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9
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1
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9
0
1
2
3
4
5
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7
8
9
0
1
3.44 ± 0.22
221 ± 52
3.59 ± 0.22
fitted 3.72 ± 0.10 2.47 ± 0.70
0.45± 0.03 4.15 ± 0.11
0.46 ± 0.03
a
b c
Reaction order with respect to bulk stannane concentration c. Limiting film thickness. Apparent
rate constant, which contains the pre-equilibrium constant from equation (1).
The Alkyls Transferred. The identity of the groups transferred to the gold surface was
established by recording the infrared spectra of the monolayers using photomodulation infrared
reflection-absorptionspectroscopy(PM-IRRAS) with carefulbackgroundsubtraction. Thepresence
of background is hard to avoid when working under ambient conditions and we rely primarily on
isotopic labeling of the alkyls present in the stannane to differentiate them from atmospheric
impurities. Even though we use polarization modulation to minimize interference by isotropic
impurities, some of thebackgroundmayalso be due to C-H vibrations of atmospheric impurities that
collect on spectrometer windows and mirrors, if they are partially oriented relative to the surface
normal and the light is not incident exactly along the normal.
The assignments of peaks to CH , CH , CD , and CD C-H and C-D stretching vibrations
2 3 2 3
4
6,47,48
-1
An additional strong peak or shoulder at ~2898 cm appeared in the
followed the literature.
ordinary IR spectra of all stannanes 1 - 15 if and only if they contained an SnCH group and seems
2
to be associated with a perturbed CH vibration in analogy to a similar perturbed CH Si
2 2
49
antisymmetric stretch vibration of silanes. Interestingly, an analogous peak is not present in the
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C-D stretching region, suggesting that it is not due to a fundamental vibration but most likely to a
Fermi resonance. However, at the present moment we cannot exclude either possibility with
-1
certainty and the association of an intense peak or shoulder at ~2898 cm with the presence of the
10
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SnCH2 moiety in the molecule is strictly empirical. Only very weak absorptions were observed in
the bending region and we were not able to use them for analysis. Weak vibrations in the C=O and
C-O stretching regions in the spectra of the films appeared in the crude spectra but were absent
within the experimental error after base line subtraction.
A 3-h immersion in THF-d alone, followed by rinsing with THF-d and drying, generates
8 8
weak peaks attributable to CD and CD groups, whose origin must be the solvent. The species
2 3
responsible for these absorptions is or areadsorbed onlyweaklyand subsequent immersion in a THF
solution of an undeuteriated stannane removes them entirely. These C-D peaks do not form when
-4
the gold surface is immersed in a 1 × 10 M solution of 6 in THF-d , and only strong signals due to
8
the CH and CH groups derived from 6 are present (Figure S1). Similarly, an initial immersion of
2 3
the gold surfacein THF for a few hours followed bydrying does not affect the subsequent deposition
of a monolayer from a solution of 2.
-4
The spectra of gold surfaces treated with 1×10 M THF or CHCl solutions of 1 for up to 18
3
-1
hours showed no detectable intensity for any C-D stretching vibrations in the 2000 to 2300 cm
-3
region. Even at a concentration of 1×10 M in THF or CHCl , no C-D stretching intensity was
3
observedafter18 h. We estimate from the signal-to-noise ratio obtained in the C-D stretchingregion
for the monolayer produced with 3 that a few per cent of a monolayer similar to the others would
have been detectable for 1, and conclude that under these conditions methyl groups are not being
transferred to the gold surface to any significant extent.
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6
-4
When a gold surface is immersed in a 1×10 M solution of 2 in THF or CHCl for 5 hours,
3
no C-D stretching intensity is observed on the rinsed and dried samples, but after 18 hours it appears
-4
clearly. When 5×10 M solutions of 2 in either solvent are used, C-D stretching intensity is
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obviously present already after 3 hours of immersion. We have noted that the spectral shape in the
region of the asymmetric CD stretch in these samples changes during extended immersion in the
3
stannane solution in THF and intend to investigate this in detail at a future time.
The spectrum of the monolayer producedfrom 3 in THF exhibited four main peaks in the CH
and CD stretching regions, attributable to CH and CD groups (Figure 4), and leaves no doubt that
2 3
the propyl group has been transferred to the surface. For comparison, Figure 4 also provides the
FTIR-ATR spectrum of pure 3, and it is apparent that the intensity of the í (CD ) peak relative to the
s 3
í (CD ) peak is enhanced in the spectrum of the monolayer, suggesting that the C-CD bond is
as 3 3
inclined toward the surface normal. However, relative integrated intensities do not confirm this
trend within experimental error and a reliable conclusion is not possible. No particular alignment
is observed for the methylene groups. The peaks attributable to í(CH - Sn) in the spectrum of neat
2
3
and marked in red in Figure 4 are absent in the spectrum of the monolayer. The IR spectra
obtained from 3 in CHCl solution were the same as those obtained from THF solution.
3
The PM-IRRAS spectra of monolayers obtained from THF solutions of 5, 6, and 10 on an
Au substrate are displayed in Figure 5 along with the spectra of neat 5 and 6. The spectra of 5 and
of the monolayer formed from 5 show only vibrations attributable to the CD and CD groups and
3
2
negligible intensity for CH vibrations that might have arisen from the solvent or from extraneous
impurities. The spectra of the monolayers formed from 6 and 10 show CH and CH stretching
3
2
vibrations. Together, the spectra demonstrate that the butyl groups are transferred to the surface.
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Figure 4. FTIR-ATR spectrum of neat 3 (orange) and PM-IRRAS
spectrum of the monolayer produced from 3 on Au substrate
(
green). FR indicates a Fermi resonance.
Figure 5. FTIR-ATR spectra of neat 5 (orange) and 6
(
blue) and PM-IRRAS spectra of monolayers produced
from 5 (green), 6 (violet) and 10 (red) on cleaned Au
substrates.
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5
6
The absence of CD vibrations in the case of the monolayer produced from 10 reveals that the methyl
groups that are also attached to the tin atom of 10 are not transferred to the surface. The CH Sn
2
vibrations contribute significantlyto the spectrum of neat 6 but not to the spectrum of the monolayer
produced from 6 and this is compatible with the notion that the butyl group is transferred without
the tin atom. The differences in relative peak intensities in the spectra of neat compounds and the
monolayers are difficult to discern and interpret due to excessive band overlap.
0
1
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1
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8
9
0
The spectra obtained from THF solutions of 2, 4, 7 - 9, and 11 - 15 are provided in Figure
S2 and are very similar to those shown here. For 2, 3 and 12, a CHCl solution was also used and
3
producedthe same IR spectra. In particular, the spectrum of the film obtained from a CHCl solution
3
of 12 contained no C-D stretching intensity, demonstrating that even in this solvent methyl groups
are not transferred to the surface. The spectrum of 15 does not contain the peaks expected for the
-1
-1 -
tosyloxyor tosylate groups, such as 3100 cm , í(C-H, aromatic), 1260cm [í (SO )], and 1105 cm
a
-1
3
-
[í (SO )].
3
48
s
ThepeakfrequenciesobservedformonolayersformedfromTHFsolutions of1-15arelisted
in Table 3. If we assume that isotopic substitution has a negligible effect on the chemical properties
of the stannanes (Figure 2B), inspection of the results contained in this Table reveals the following:
(
i) Methyl groups are never transferred from a stannane to the Au surface, and ethyl groups are
transferredonlyveryslowly. (ii) n-Propyl and n-butyl groups are always transferredfrom a stannane
to the Au surface. (iii) The alkyls that have been transferred to the Au surface are no longer attached
to tin.
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-1
Table 3. CH and CD stretching vibrations observed in the PM-IRRAS of Au surfaces treated with THF solutions of 3 - 15 (cm ).
a
group mode
CH
2
-
3
-
4
5
6
7
8
9
10
2878 2878 2877 2879 2878 2879 2879
2961 2964 2964 2962 2963 2965 2965
11
12
13
×
14
-
15
2879 2878
2960 2964
16
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
í
2877
-
-
3
s
í
-
-
2963
×
-
as
CD
í
2075 2075
2131
2216 2212
- 2865 2856
-
-
-
-
2074
2132
2220
-
-
-
-
-
-
-
×
×
×
-
-
-
×
×
×
-
-
-
×
×
×
2072 2073
2135 2134
2220 2219
-
-
-
-
-
-
3
s
í (FR)
×
í
as
CH
í
2863 2862 2858 2863 2860 2862 2862
-
-
-
-
-
-
2863 2851
2
s
í (CH -Sn)
2
×
×
-
×
×
×
×
×
×
×
×
-
í
-
2925 2926
- -
- -
-
2928 2930 2929 2929 2925 2929 2929
- - - - - - -
- - - - - - -
2928 2920
- -
- -
as
CD
í
2186
2149
2103
2205
2103 2102
2206 2203
2
s
í
as
a
No IR peaks were observed for 1. Significance of symbols: -, this isotope is absent in the stannane used; ×, this isotope is present in the stannane used, but no
vibration attributable to it was detected in the spectrum of the monolayer.
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6
SurfaceLayerElementalComposition. X-rayphotoelectronspectra(XPS)wererecorded
for monolayers formed from ten of the stannanes. Under exposure to X-rays in ultrahigh vacuum
~
15% of the monolayer can desorb as indicated by a reduction of IR intensity of C-D stretching
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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7
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0
vibrations of films deposited from THF solutions of 5 and 14 measured before and after the XPS
measurement. A mild increase of intensity of C-H vibrations was also noticed, likely originating
from contaminating species adsorbed from the ambient during transport of samples. Illustrative
spectra are displayed in Figures 6 and 7 and the others are shown in Figures S3 and S4. The
measured surface photoemission intensity ratios adjusted by Scofield photoionization cross-
50
sections along with the surface concentration of tin calculated assuming that all of it is located in
the very first surface layer and exposed to vacuum are displayed in Table 4.
Table 4. Sn 3d/Au 4f, C 1s/Au 4f, O 1s/Au 4f, and C 1s/Sn 3d photoemission intensity ratios
a
adjusted by Scofield photoionization cross-sections.
b
Sample Reagent
Sn/Au C/Au O/Au C/Sn c(Sn)
c
0
1
2
3
6
7
8
THF
0
0.9
0.28
0.17
0.41
0.31
0.18
0.40
0.46
0.16
0.11
0.38
0.27
0.20
0.27
0.16
-
0
Sn(CD )
3
0.04
0.51
0.95
0.77
0.55
0.71
0.79
0.41
0.26
0.39
0.53
0.53
0.70
0.46
12.8
11.8
9.2
2.3
4
Sn(C D )
2
0.076
0.084
0.036
0.087
0.087
0.107
0.024
0.18
4.4
4.7
2.3
4.8
4.8
6.1
1.4
10.1
1.4
3.9
5.8
1.3
5 4
Sn(C H D )
3 4
3 4
Sn(C H )
4
15.3
8.2
9 4
Sn(C H ) CH
4 9 3
3
3
Sn(C H ) CD
4 9 3
9.1
9
Sn(C H ) (CH )
4 9 2
3.8
3 2
d
9
Sn(C H ) (CH )
4 9 2
10.8
2.2
3 2
e
9
Sn(C H ) (CH )
4 9 2
3 2
1
0
Sn(C H )(CD )
4 9
0.024
0.067
0.104
0.023
22
3 3
11
Sn(C H )(CH )
4 9
7.9
3 3
1
3
Sn(C D )(CH )
4 9
6.7
3 3
1
5
SnBu CH OTs
2
3
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a
-4
Monolayers formed by treatment of cleaned gold surface with a 5 × 10 M solution of a stannane in THF for 4 h.
b 14 2
Estimated accuracy, ±10%. Surface concentration of tin atoms in units of 10 atoms/cm . The value for a tin
c
monolayer is 11.1. Flame annealed gold film on mica immersed in THF for 4 hours without any stannane added.
d
e +
Adsorption from gas phase on template stripped (Platypus) gold substrate. Au surface cleaned by mild Ar sputtering
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(
E = 4 keV, I= 10 uA, t = 10 min).
Figure 6. XPS of layers on Au surface. (A) C 1s, cleaned Au
surface immersed for 4 h in pure THF. (B) C 1s, (C) O 1s, and
(D), Sn 3d for monolayers formed by treatment of a cleaned gold
-4
surface with a 5 × 10 M THF solution of 11.
2
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Figure 7. XPS of O 1s photoelectrons from a cleaned gold surface
-4
treated with a 5 × 10 M THF solution of 11 measured at two
different detection angles (defined from the sample surface).
The XPS results revealed the presence of tin in the monolayers prepared from the stannanes.
The binding energy obtained for Sn 3d photoelectrons, 486.5 ± 0.1 eV, was identical for all
5/2
samples and was consistent with the presence of tin oxide on the gold surface. This assignment is
corroborated by the presence of a component with binding energy of 530.3 eV in the spectra of O
51
s photoelectrons, which is characteristic of metal oxides, including tin oxide.
52,53
1
The Sn 3d
5
/2
4+ 2+
binding energy shift between Sn and Sn formal valencies in tin oxides is rather small
53,54
and thus
the measured binding energy does not allow unambiguous differentiation between SnO, SnO or
2
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some intermediate stoichiometry. In addition, the available values were measured on bulk oxides
and may differ from those for SnO nanostructures that can be expected in our samples.
x
The energy separation between the O 1s component belonging to Sn oxide and the Sn 3d
5
/2
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5
4,55
peak was used bysome authors
to distinguish betweenSnO and SnO , even though the difference
2
between the two is small (0.1 eV). Our value, 43.9 eV, fits those reported for SnO . It needs to be
2
56
noted that the binding energy of Sn 3d electrons published for Bu Sn, 486.3 eV, lies within the
4
5
/2
experimental error of values reported for tin oxides.
The spectra of O 1s photoelectrons can be fitted by two components centered at 530.3 and
531.8 eV (see Figure 7 and Figure S4). The former is attributable to tin oxide, and after correction
for photoionization cross-sections its integrated intensity relative to that of Sn 3d emission yielded
an atomic concentration ratio O/Sn = 1.5 ± 0.2, indicating that SnO, SnO and/or a suboxide might
2
all be present on the Au surface. The high binding energy component of the O 1s peak is due mainly
5
4
to oxygen in C–O, O–C=O, and –OH groups, which have similar binding energies and cannot be
distinguished. This spectral component is also present in the spectra of gold samples after their
immersion in pure THF and subsequent drying, and its intensity depends on the method used for the
initial gold surface cleaning. The ratio of intensities of components of the O 1s spectra is different
for different samples and is a function of the detection angle of photoelectrons (Figure 7). The
relative weight of the higher binding energy O 1s component increases as the detection angle
measured from the sample surface decreases, showing that the responsible oxygen containing
functionalities are more distant from the gold surface than the oxygen atoms belonging to tin oxide.
This finding indicates that these species are present (adsorbed) on top of the initial monolayer. They
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likely originate primarily from adventitious contamination or oxidation of samples exposed for
certain time to ambient air.
The C 1s spectra revealed that a carbonaceous contamination carrying small amounts of
singly and doubly bonded oxygen functionalities is present even on a gold surface immersed in pure
THF solvent containing no stannanes, yielding a C/Au ratio close to 1. This carbon species is
0
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0
presumablyalso responsible for the weak IR intensityin the C-D stretch region when THF-d is used
8
(Figure S1). As noted above, most or all of this species is removed during the process of surface
alkylation with a stannane.
For the monolayers prepared from stannane solutions, the C/Au ratio varies from sample to
sample within the range 0.2 - 1. In most instances it is smaller than 1, suggesting that in the presence
of a stannane the amount of carbonaceous impurity deposited from the solvent is indeed reduced,
and that it does not represent a stable background that could be subtracted. This prevents us from
using the C/Au ratio to deduce the surface concentration of the alkyl groups, even if one could
correct for the attenuation of the Au signals by an overlayer that contains variable amounts of tin.
The core level binding energy of the main component of C 1s spectra obtained for surfaces
with adsorbed stannanes is 284.2 ± 0.2 eV and can be assigned to carbon atoms screened by gold
substrate electrons. This value is considerablylower than that measured for the adventitious carbon
impurity on gold, 284.8 - 285.0 eV. This difference might be accounted for by extra-atomic
relaxation occurring with participation of gold electrons, which is possible for carbon atoms in the
short hydrocarbon chains that are close to the gold surface. A continuous shift of C 1s binding
40
energy with a growing length of the carbon chain has already been reported for self-assembled
alkanethiol layers on gold surfaces.
2
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+
Adsorption of9 fromsolution wasstudied also on Au surfacesputtered byAr ions. Surfaces
57
cleaned in this way are known to contain considerable concentration of various defects. In this
experiment, the sputtered sample was taken into the open under flow of N for about 10 s and
2
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immersed in a THF solution of 9, followed by the usual rinsing with pure THF, drying in a stream
of N , and insertion into the spectrometer with an about 15 s exposure to ambient atmosphere. This
2
procedure produced about a monolayer of tin oxide and a low surface concentration of carbon (C/Sn
=
2.2). This result demonstrates that surface defects play a significant role in adsorption and
subsequent surface reactions of stannanes.
In another experiment in which a clean Au surface was exposed for 4 h to the vapor of 9 at
room temperature, the C/Sn stoichiometry corresponded well to the molecular formula but the
surface concentration of both carbon and tin was low, likely due to low vapor pressure of the
stannane used. The presence of the component with binding energy 530.4 eV belonging to tin oxide
in the spectrum of O 1s photoelectrons seems to exclude the possibility that molecules of 9 are
merely molecularly adsorbed.
An attempt was made to use room-temperaturescanningtunneling microscopy(STM) under
ambient conditions to find the possibly present islands of tin oxide on the sample surface, but all
images looked like those of cleaned gold. It is likely that the tin oxide is not segregated into large
domains but is interspersed with the alkyls on the surface.
Monolayer Blocking Properties. Information on the permeability of the monolayers was
sought from an examination of cyclic voltammograms (CV) and electrochemical impedance spectra
3-
EIS) of 2 mM [Fe(CN) ] in aqueous 0.1 M KNO in a four-electrode electrochemical cell at a scan
3
(
6
-1
-4
rate of 7 mVs . We tested monolayers obtained after a 5-h treatment of a gold electrode with 10
2
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M THF solutions of 1, 2, 4, 6, 9 and 15. The EIS curves are shown in Figure 8. A quantitative
evaluation produced the monolayer charge transfer resistance values R listed in Table 5.
ct
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3-/4-
Figure 8. CV (A) and EIS (B) responses to [Fe(CN) ] observed
6
on the cleaned Au substrate (0) and monolayers produced from
THF solutions of 1, 2, 4, 6, 9, 15 and 16.
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The CV results agree with those of EIS, but the latter are a much finer tool for evaluating the
3-/4-
access that the [Fe(CN) ] ions have to the conducting surface, either gold or tin oxide. The results
6
obtained after 5-h treatment of the gold electrode surface with THF solutions of 1 and 2 are nearly
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identical with those obtained on the cleaned gold surface, in line with the above conclusion that no
-4
insulating alkyl groups are deposited on the gold in a few hours from 10 M solutions in these two
cases, particularly not from 1. The slight difference with respect to the cleaned gold surface that is
observed most clearly in Table 5 is attributed to the presence of a well-conducting tin oxide layer
on the surface treated with the stannane. The resistance R of the propylated surface obtained with
ct
4 is twice higher, and that of the butylated surface obtained with 6, 9, and 15 is five to six times
higher. All of these values are orders of magnitude lower than the resistance of the monolayer we
-2
obtained with the long-chain thiol 16, 3.4 kOhm.cm . The latter agrees well with the reported value
-2
of 3.6 kOhm.cm for a monolayer of decanethiol.
58
-4
Table 5. Charge transfer resistance R of monolayers deposited from 10 M THF solutions after
ct
a
5
h.
-2
Compd.
R / Ohm.cm
ct
0
(bare Au)
13
20
1
2
4
6
9
29
65
127
94
1
5
125
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a
Estimated error: ~±5%.
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Monolayer Stability. The IR spectra of the monolayers obtained after a 4-h immersion in
-
4
a 5×10 M THF solution of 2 or 5 (Figure S5) became about 10% weaker but otherwise did not
change significantly after subsequent 18-h immersion in pure THF. In the case of 2, this was tested
both in THF and in chloroform, and no change was observed. However, extended immersion in a
-4
5×10 M solution of 2 in THF caused a spectral change in the region of asymmetric CD vibration.
3
Subsequent immersion in pure THF had no further effect.
Figure 9 shows that the thermal stability of the monolayers formed from 4 and 5 is
comparable to that of a monolayer formed from n-C H SH (16), both under ambient conditions
18 37
(
part A) and under reduced pressure at 200 EC (part B). We have already mentioned a slight degree
of removal of the monolayers during an XPS measurement in ultrahigh vacuum. In all cases, the
amount of monolayer present was estimated from the integrated IR intensity in the CH or CD
stretching regions.
-4
Upon treatment with a 1×10 M solution of C H SH in THF for 18 hours, a film produced
37
1
8
from a solution of 5 in THF was completely replaced with a self-assembled monolayer of the
alkanethiol, as judged both by the IR spectrum (Figure S6) and an electrode blocking measurement.
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Figure 9. Thermal stability of monolayers deposited from 4
(
orange), 5 (violet), and n-C H SH (16, gray). R is the per cent
18 37
of monolayer remaining, evaluated from the ratio of final to initial
integrated IR intensity in the 2800 - 3000 (4, 16) or 2000 - 2300 (5)
-1
cm region. (A) In laboratory air, 1 h at each temperature. (B) At
2
00 EC, under 1.5 mbar pressure.
2
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