Zwitterionic dithiocarboxylates derived from N-heterocyclic carbenes: Coordination to gold surfaces

Article abstract of DOI:10.1039/c2dt11976e

The zwitterionic dithiocarboxylates 1⁺-CS₂ ^--4⁺-CS₂^- were prepared by reacting the corresponding N-heterocyclic carbenes 1,3-bis(2,6-diisoproylphenyl)imidazol- 2-ylidene (1), 1,3-diisopropylimidazol-2-ylidene (2), 1,3-dibenzylimidazol-2- ylidene (3) and 1,3-diethylbenzimidazol-2-ylidene (4) with CS₂. In the latter two cases, the corresponding N-heterocylic carbene was generated in situ. Compounds 2⁺-CS₂^--4⁺-CS ₂^- were structurally characterised by single-crystal X-ray diffraction studies. The chemisorption of these zwitterionic dithiocarboxylates on solid gold substrates was investigated in situ and in real time by optical second harmonic generation (SHG). The resulting thin films were exemplarily characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the case of 1⁺-CS ₂^- and 2⁺-CS₂^-, revealing the formation of almost contamination-free self-assembled monolayers, which exhibit a remarkable degree of orientational order.

Full text of DOI:10.1039/c2dt11976e

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PAPER
Zwitterionic dithiocarboxylates derived from N-heterocyclic carbenes:
coordination to gold surfaces†
Ulrich Siemeling,*a,c Henry Memczak,‡a,c Clemens Bruhn,a Florian Vogel,b,c Frank Träger,b,c Joe E. Baiod
and Tobias Weidnerd
Received 19th October 2011, Accepted 5th December 2011
DOI: 10.1039/c2dt11976e
The zwitterionic dithiocarboxylates 1+–CS2−–4+–CS2− were prepared by reacting the corresponding N-
heterocyclic carbenes 1,3-bis(2,6-diisoproylphenyl)imidazol-2-ylidene (1), 1,3-diisopropylimidazol-2-
ylidene (2), 1,3-dibenzylimidazol-2-ylidene (3) and 1,3-diethylbenzimidazol-2-ylidene (4) with CS2. In
the latter two cases, the corresponding N-heterocylic carbene was generated in situ. Compounds 2+–
CS2−–4+–CS2− were structurally characterised by single-crystal X-ray diffraction studies. The
chemisorption of these zwitterionic dithiocarboxylates on solid gold substrates was investigated in situ
and in real time by optical second harmonic generation (SHG). The resulting thin films were exemplarily
characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray
photoelectron spectroscopy (XPS) in the case of 1+–CS2− and 2+–CS2−, revealing the formation of almost
contamination-free self-assembled monolayers, which exhibit a remarkable degree of orientational order.
is part of our programme addressing the multipoint attachment
Introduction
of bi-8 and oligodentate9 adsorbate species on gold. We note that
Owing to their high nucleophilicity, N-heterocyclic carbenes
Lee and coworkers have shown that dithiocarboxylic acids
R-CS2H (R = n-alkyl) form self-assembled monolayers (SAMs)
on gold.10 Such SAMs contain surface-bound anionic dithiocar-
boxylate groups R–CS2−, while SAMs fabricated from zwitter-
(NHCs) react with carbon disulfide, affording zwitterionic
− 1
dithiocarboxylates of the general type NHC+–CS2
.
First
examples of such compounds had been prepared less straightfor-
wardly from enetetramines and CS2 already several decades
ago.2 These pseudo-cross-conjugated mesomeric betaines3 are
currently attracting great interest. Their potential for probing the
stereoelectronic parameters of NHCs was recently investigated
by Delaude et al.4 They have been utilised as efficient organo-
catalysts,5 and their application as ligands, which is based on
sporadic work in the 1980s,6 is a highly dynamic area of tran-
sition-metal coordination chemistry.7 Very recently, Delaude,
Wilton-Ely and co-workers have demonstrated that zwitterionic
−
ionic dithiocarboxylates NHC+–CS2 are expected to contain
intact, neutral adsorbate molecules. In general, zwitterionic
ligands are an attractive alternative to anionic ligands like thio-
lates, because they can become attached to metallic substrates
without a change of the oxidation state of the surface metal
atoms.
Results and discussion
dithiocarboxylates NHC+–CS2 are also suitable for the stabilis-
−
Synthetic work and crystal structures
ation of gold nanoparticles.7c This has prompted us to study their
chemisorption on solid gold substrates. The present investigation
The nucleophilic addition reactions of the NHCs 1–4 with an
excess of CS2 in THF afforded the corresponding zwitterionic
dithiocarboxylates 1+–CS2−–4+–CS2− as red solids in good yield
(Scheme 1). While the 2,6-diisopropylphenyl-substituted imida-
zol-2-ylidene 1 and the isopropyl-substituted imidazol-2-ylidene
2 were synthesised and isolated as crystalline solids by well-
established methods,11 the benzyl-substituted imidazol-2-ylidene
3 and the ethyl-substituted benzimidazol-2-ylidene 4 were gener-
ated for this purpose in situ by deprotonation of [3H]Cl12 and
aInstitute of Chemistry, University of Kassel, Heinrich-Plett-Str. 40,
34132 Kassel, Germany. E-mail: siemeling@uni-kassel.de
bInstitute of Physics, University of Kassel, Heinrich-Plett-Str. 40, 34132
Kassel, Germany
cCenter for Interdisciplinary Nanostructure Science and Technology,
University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany
dNational ESCA and Surface Analysis Center for Biomedical Problems
(NESAC/BIO), Departments of Bioengineering and Chemical
Engineering, University of Washington, Seattle, Washington 98195, USA
†Electronic supplementary information (ESI) available: Fig. S1 showing
the pleochroitic behaviour of compound 3+–CS2−. CCDC reference
numbers 847616–847618. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt11976e
[4H]Br,13 respectively. 1+–CS2 had already been obtained pre-
−
viously by Delaude et al. in lower yields by the in situ method
and was characterised crystallographically by these authors.4
−
Although 2+–CS2 was used as a ligand in a recent study, pre-
parative and analytical details have been unavailable for this
‡Current address: Fraunhofer Institute for Biomedical Engineering, Am
Mühlenberg 13, 14476 Potsdam-Golm, Germany.
compound.7a,c
2986 | Dalton Trans., 2012, 41, 2986–2994
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Fig. 2 Molecular structure of 3+–CS2 in the crystal. Only one of the
two independent molecules is shown. Thermal ellipsoids are drawn at
the 30% probability level.
−
CS2−, which does not show the approximately orthogonal orien-
tation of the CS2 and CN2 planes which is usually found for
such zwitterionic compounds. The two independent molecules
Scheme 1 Synthesis of the zwitterionic dithiocarboxylates for 1+–
−
of 3+–CS2 exhibit a deviation from orthogonality of ca. 20°
CS2−–4+–CS2
.
−
and ca. 30°, respectively, as is indicated by their N–C–C–S
torsion angle values (molecule 1: 71.55°, −109.25°, −107.39°,
71.81°; molecule 2: −60.10°, 121.22°, 118.62°, −60.06°).
All compounds were further characterised by pertinent spec-
troscopic methods. Their characteristic red colour originates from
CS2−-based n→π* transitions.6b,14 Optical spectroscopy revealed
that the corresponding absorption maximum is located at ca.
530 nm in all cases (dichloromethane solution). Compounds 2+–
We have carried out single-crystal X-ray diffraction studies for
2+–CS2−–4+–CS2−. The molecular structures of these com-
pounds are shown in Fig. 1–3. Pertinent bond parameters are col-
lected in Table 1, which also includes the corresponding data of
−
1+–CS2 for comparison. As a sideline of our crystallographic
investigation we note that single-crystals of the benzyl-substi-
−
tuted compound 3+–CS2 exhibit strong pleochroism (see ESI,
Fig. S1†). Bond lengths and angles compare well to those of
− 1a,4
other compounds of the type NHC+–CS2
.
The only remark-
able feature occurs in the case of the benzyl-substituted 3+–
−
−
Fig. 1 Molecular structure of 2+–CS2 in the crystal. Thermal ellip-
Fig. 3 Molecular structure of 4+–CS2 in the crystal. Thermal ellip-
soids are drawn at the 30% probability level.
soids are drawn at the 30% probability level.
Table 1 Selected bond lengths (Å) and angles (°) for compounds 1+–CS2−–4+–CS2
−
Compound
C–CS2
C–S
N–CCS2
N–C–N
107.5(1)
107.0(2)
S–C–S
1+–CS2
1.487(2)
1.485(4)
1.659(5)
1.676(3)
1.675(3)
1.677(3)
1.674(5)
1.675(5)
1.674(5)
1.677(5)
1.668(5)
1.669(5)
1.341(2)
1.387(1)
1.351(3)
1.356(3)
1.337(6)
1.346(6)
1.348(6)
1.365(6)
1.340(5)
1.342(5)
128.5(2)
130.5(2)
Ref. 4
−
2+–CS2
This work
This work
−
3+–CS2
1.507(6)
1.500(6)
108.0(4)
107.0(4)
129.8(3)
129.0(3)
−
2 molecules
4+–CS2
1.499(6)
109.5(4)
130.6(3)
This work
−
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CS2−–4+–CS2−, which carry aliphatic N-substituents, exhibit a
second n→π* absorption at slightly higher energy (λmax ca.
440 nm). In addition to these fairly weak bands (logε ca. 2) in
−
the visible region the CS2 group gives rise to an intense π→π*
absorption in the UV region (λmax ca. 360 nm, logε ca. 4). The
ring systems are responsible for π→π* bands of similar intensity
(logε ca. 4) at higher energy. While two absorption maxima
located at ca. 235 and 260 nm are observed for the imidazolium-
based compounds 1+–CS2−–3+–CS2−, the benzimidazolium-
−
based derivative 4+–CS2 exhibits three π→π* absorptions
(λmax = 229, 273, 309 nm).
−
+
The charge delocalisation in the CS2 and CN2 parts of the
zwitterions, as indicated already by the essentially identical
carbon–heteroatom bond lengths in each unit (Table 1), is nicely
reflected by characteristic data from vibrational spectroscopy.
The highest intensity band in the Raman spectra is located in the
narrow range between ca. 1465 and 1490 cm−1 for these com-
Fig. 4 SHG signal recorded in situ and in real time during the adsorp-
tion of 1+–CS2 onto a gold substrate from a 100 μmol L−1 (black
−
curve) and a 50 μmol L−1 dichloromethane solution (grey curve),
respectively.
+
pounds and can be attributed to the νs(CN2 ) vibrational mode.
+
The corresponding νas(CN2 ) vibrational band was observed
between ca. 1465 and 1500 cm−1 in the IR spectra, which
further exhibit the characteristic νas(CS2−) vibrational band
between ca. 1045 and 1060 cm−1, in line with previously
for a wide variety of sulfur-based adsorbate species such as, for
example, thiols,17 thioethers,9d,18 disulfides18 and dithiolane
derivatives.8b This behaviour can be explained by a decrease of
the number of free electrons at the surface of the gold substrate
by localisation in chemical bonds between the gold atoms and
the adsorbing molecules. SHG is not suitable for probing the
nature of these bonds on the surface. This can be achieved,
however, by XPS and NEXAFS spectroscopy (vide infra).
−
reported experimental findings for 1+–CS2 and analogous com-
pounds4 and results of ab initio calculations.15
+
The CN2 and CS2− units give rise to 13C NMR signals at ca.
150 ppm and 220–225 ppm, respectively, which agrees very well
with data reported for closely related compounds.1d,4
From the SHG data, we can calculate the coverage Θ of adsor-
bate molecules on the gold surface. If the SHG signal is satu-
rated, the surface coverage is defined as Θ = 1. The temporal
development of the surface coverage Θ is shown in Fig. 5a for
c = 100 μmol L−1 and in Fig. 5d for c = 50 μmol L−1. The
adsorption kinetics can now be compared to three kinetic models
which have been proposed in earlier studies to describe the for-
mation of SAMs: (i) the first-order Langmuir model (FO), i. e.
direct chemisorption of the molecules from the solution;19 (ii)
the second-order non-diffusion limited model (SO) involving the
combined chemisorption of two groups present in one mol-
ecule;20 (iii) the diffusion-limited Langmuir model (DL), i.e. dif-
fusion of the molecules from the solution to the surface must be
taken into account.19a Fig. 5b–c displays the agreement of the
kinetic models with the surface coverage data for c = 100 μmol
L−1. For c = 50 μmol L−1, this is shown in Fig. 5e–f. The abscis-
sae are chosen in such a way that the experimental coverage data
should follow a straight line for the respective kinetic model. For
c = 100 μmol L−1, all kinetic models show small deviations
from the experimental data (cf. Fig. 5b–c). The best fit occurs
with the SO model, which is expected to be particularly suitable
SAM fabrication and investigation of the chemisorption process
by optical second harmonic generation (SHG)
−
SAMs of 1+–CS2−–4+–CS2 were prepared from solution on
solid gold substrates by applying a standard protocol (dichloro-
methane, room temperature, immersion time ca. 20 h). The
SAM formation process was investigated in situ and in real time
by optical second harmonic generation (SHG).16 The SHG
signal of the clean substrate in pure dichloromethane was
recorded for reference purposes. Subsequently, the pure solvent
was replaced by a dichloromethane solution of the adsorbate
species, and the SHG signal was instantly monitored in situ as a
function of time. The results of this SHG study are described
here exemplarily for 1+–CS2−. The other compounds showed
similar behaviour. Fig. 4 shows the real-time SHG signal of the
adsorption of this compound on the gold substrate for two differ-
ent concentrations, viz. c = 50 μmol L−1 and c = 100 μmol L−1
.
At t < 0 the signal of the plain gold substrate immersed in pure
solvent is measured. This signal has been set to unity. The
change of the pure solvent to the solution of the adsorbate
species causes an abrupt decrease of the SHG signal at t = 0 and
subsequently a decrease of the SHG signal is observed until sat-
uration is reached.
−
for the theoretical description of the adsorption of 1+–CS2 on
gold surfaces, since two binding events can occur with the
bidentate dithiocarboxylate headgroup. Obviously, for a concen-
tration of c = 50 μmol L−1 the DL model fails to describe the
experimental data. The FO model shows much smaller devi-
ations from the data, but again the SO model is most consistent
with the measurements. The rate-limiting step for the adsorption
The SHG data indicate similar adsorption behaviour for both
concentrations. Not surprisingly, the adsorption process takes
slightly longer for c = 50 μmol L−1 than for c = 100 μmol L−1
,
as is indicated by the time needed to reach saturation of the SHG
signal in each case, viz. t = (3500 500) s for c = 100 μmol L−1
and t = (4500 500) s for c = 50 μmol L−1. In both cases a
drop-off of the intensity of the SHG signal to 0.80 0.02 is
observed. A decrease of the SHG signal was also noticed before
−
of 1+–CS2 on gold surfaces therefore appears to be the for-
mation of two gold–sulfur bonds between the headgroup and the
surface atoms.
2988 | Dalton Trans., 2012, 41, 2986–2994
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Fig. 5 Coverage of 1+–CS2− on the gold substrate as a function of time for a concentration of 100 μmol L−1 (a) and 50 μmol L−1 (d). Coverage as a
function of the kinetic model for 100 μmol L−1 (b and c) and 50 μmol L−1 (e and f); the abscissae are chosen such that for the corresponding model
the coverage data should follow a straight line.
SAM characterisation
compositions fall into agreement with the stoichiometry of the
two adsorbates. The only significant differences between theor-
etical and experimental compositions are that the atomic percen-
tages of S and N are lower than expected (3.8 versus 14.3 sulfur
at% in the case of 2+–CS2−). If well-ordered monolayers are
formed the sulfur and nitrogen containing groups are close to the
gold surface and covered by the rest of the molecule. Thus, these
lower percentages can be explained by attenuation of the sulfur
and nitrogen signals due to inelastic scattering.
Fig. 6 shows high-resolution S 2p, C 1 s and N 1 s XP spectra
for both SAMs on Au. The C 1 s spectra consist of two features,
one at 284.9 eV corresponding to the alkyl chains and aromatic
carbon rings and a high-energy shoulder at 286.0 eV that can be
XPS results. Surface characterisation was exemplarily per-
formed for SAMs fabricated from compounds 1+–CS2 and 2+–
−
CS2−, respectively. Elemental compositions of these SAMs were
determined by X-ray photoelectron spectroscopy (XPS), which
showed the presence of carbon, nitrogen, sulfur, oxygen, and
gold (from the underlying substrate). The oxygen observed at
the surface can be attributed to minimal substrate contamination
prior to the film assembly, since no significant oxidation was
indicated by the high-resolution XP spectra or the near-edge X-
ray absorption fine structure (NEXAFS) data discussed later.
Once the gold contribution is omitted (Table 2) the experimental
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Table 2 Summary of XPS determined elemental compositionsa for thin films of 1+–CS2− and 2+–CS2− on gold
Au
C
N
S
O
−
−
1+–CS2
Theor. comp.
Exp. comp.
Exp. comp. w/o Au
Theor. comp.
Exp. comp.
0
87.5
6.25
6.25
0
65.4(2.8)
0
0
59.9(2.3)
0
19.9(1.9)
81.7(2.1)
71.4
33.1(0.9)
82.5(2.2)
1.9(0.6)
5.5(1.5)
14.3
3.2(0.8)
8.0(2.0)
2.2(0.3)
6.4(1.3)
14.3
3.8(0.3)
9.5(0.7)
2.2(0.9)
6.4(2.1)
0
2+–CS2
n. d.b
n. d.b
Exp. comp. w/o Au
a Values in at% with experimental errors in parentheses. b Not detected.
closely related dithiocarbamates, to a monodentate surface
binding with one sulfur atom bound to a counter ion.24 Since no
such counter ions have been detected with XPS, the BE differ-
ence between these two doublets is most likely the result of
different hybridisation states or binding geometries of a bidentate
binding configuration.9d This hypothesis is supported by results
from the molecular coordination chemistry of dithiocarboxy-
lates.25 Several binding modes have been observed for structu-
rally characterised AuI compounds, viz. monodentate,26 chelating
bidentate26a and bridging bidentate.27 However, homoleptic
complexes exclusively exhibit bridging bidentate dithiocarboxy-
lato ligands, which strongly supports the notion that this is the
preferred binding configuration in the case of dithiocarboxylate-
based SAMs on gold, as has already been proposed by Lee and
coworkers.10c,d
Fig. 6 High-resolution S 2p (left), C 1 s (middle) and N 1 s (right) XP
spectra of SAMS fabricated from 1+–CS2 (top) and 2+–CS2 (bottom)
−
−
on gold.
The effective film thickness of the SAMs was determined
from the intensities of the C 1 s and the Au 4f emissions. The
thickness values were derived from the IC1s/IAu4f intensity ratios.
We calculated thickness values of 10.0 and 9.5 Å, respectively,
assigned to the C–N and C–S groups.21 The broader width of
the shoulder, as compared to the C–C emission, may also reflect
contributions from small amounts of C–O contaminations of the
substrate. This shoulder is double in size to what was expected
for the SAMs fabricated from 1+–CS2 and 2+–CS2 by using
previously reported attenuation lengths,28 this is in excellent
agreement with monolayers on gold, since the expected thick-
ness derived from the crystal structures (vide supra) is ca. 10 Å.
−
−
−
just from stoichiometry for the SAM fabricated from 2+–CS2
(Table 3). Both SAMs contain only small amounts of oxygen,
viz. 2–7 at%, which is similar to the oxygen content of the
adventitious carbon layer we typically observe on our gold sub-
strates (ca. 5 at%) prior to adsorption.
NEXAFS spectroscopy. The molecular orientation and align-
ment in SAMs prepared from 1+–CS2−and 2+–CS2− on gold sur-
faces were studied with NEXAFS spectroscopy. This method is
extremely surface-sensitive and can provide information about
the nature and orientation of chemical bonds of adsorbate
species. Molecular orbitals are probed by monitoring resonant
transitions of atomic core electrons into unoccupied molecular
orbitals. The efficiency of the photoexcitation process is strongly
dependent on the orientation of the X-ray electric field vector
with respect to the molecular transition dipole moment (TDM).
This effect is known as the linear dichoism of X-ray absorption
and can be conveniently monitored by recording NEXAFS
spectra with p-polarised X-rays at different incidence angles.29
The N 1 s spectra of both SAMs exhibit a single feature at
401.1 eV related to the N–C bonds.21 The S 2p spectra exhibit
two S 2p3/2,1/2 doublets with binding energies (BE) of 162.2 and
161.2 eV and show no indication of oxidised sulfur species. The
ratio between the area of the peak at 162.2 eV and that of the
peak at 161.2 eV is 2 : 1 and 3 : 1, respectively, for the SAMs
−
−
fabricated from 1+–CS2 and 2+–CS2 (Table 3). These two
different doublets stem from two different sulfur species. The
peak at 162.2 eV can be clearly assigned a gold-bound thiolate
group.22 The emission near 161.2 eV has been assigned to thio-
late in a different binding geometry23 or, in the case of the
Table 3 High-resolution XPS C 1 s peak fit resultsa for thin films of 1+–CS2− and 2+–CS2− on gold
C–Hn, CvC (284.9 eV)
C–N, C–S (286.0 eV)
Thiolate S (162.2 eV)
“Different” Thiolate S (161.2 eV)
−
−
1+–CS2
Theor. comp.
Exp. comp.
Theor. comp.
Exp. comp.
87.5
73.5
71.4
42.4
12.5
26.6
28.6
57.6
64.8
73.0
35.2
27.0
2+–CS2
a Values in at%.
2990 | Dalton Trans., 2012, 41, 2986–2994
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−
Fig. 8 C K-edge NEXAFS spectra of SAMs prepared from 2+–CS2
Fig. 7 C K-edge NEXAFS spectra of SAMs prepared from 1+–CS2
−
acquired at X-ray incidence angles of 70°, 55°, and 20°. The bottom
curve represents the difference between the 70° and 20° spectra.
acquired at X-ray incidence angles of 70°, 55°, and 20°. The bottom
curve represents the difference between the 70° and 20° spectra.
average tilt angle Θ of 28° 5° for the imidazole unit. The imida-
zole tilt angle found for this SAM is slightly higher than the cor-
responding tilt angles found for related aromatic SAMs on gold,
such as, for example, SAMs based on biphenyl (Θ≈23°), para-
terphenyl (Θ ≈ 20°) or anthracene (Θ ≈ 23°) backbones,34 but it
is close to orientations observed for biphenyl tellurolate on gold
Fig. 7 and 8 show C K-edge NEXAFS spectra recorded at X-
ray incidence angles of 70°, 55° and 20° for SAMs fabricated
−
−
from 1+–CS2 and 2+–CS2 on gold. All spectra show the
expected absorption edge related to the excitation of the C 1 s
electrons into continuum states and a number of characteristic
absorption features. The pre-edge region exhibits peaks near
285.5 eV, related to C 1 s → π*(CvC) transitions of the aro-
matic rings. This resonance is significantly stronger in the case
of 1+–CS2−, which is expected in view of the additional phenyl
(Θ ≈ 28°).35 The molecular alignment is superior to SAMs with
−
short aromatic backbones comparable with 2+–CS2
, for
example benzene thiol on gold, which have been shown to form
mostly disordered layers.34
−
rings in 1. In 2+–CS2 this peak is only related to the imidazole
unit. All spectra exhibit Rydberg/C–H (R*) resonances near
288.1 eV related to the alkyl chains and broad σ* resonances
related to C–C bonds at higher photon energies. The spectra
show no signs of chemical impurities such as CvO. The above
assignments were made in accordance with ref. 9c,29–33.
Conclusion
Our results demonstrate that zwitterionic dithiocarboxylates,
which are easily accessible from readily available N-heterocyclic
carbenes in a single-step reaction with CS2, chemisorb intact on
solid gold substrates, giving rise to almost contamination-free
self-assembled monolayers, which, in view of the absence of
extended tailgroups, exhibit a remarkable degree of orientational
order. This hitherto neglected adsorbate system has therefore
great potential for complementing that of the standard gold–thio-
late-based SAMs.36
−
The spectra for 1+–CS2 show a pronounced linear dichroism
for the π* resonances, which is highlighted by the 70°–20°
difference spectrum. This is a signature of a certain degree of
orientational order and molecular alignment in the SAM. The
negative polarity of the observed difference peak implies a
strongly tilted orientation of the phenyl ring planes in 1 with
respect to the surface. Note that for 1+–CS2− the aromatic π* res-
onance is representative of both the phenyl rings and the imida-
zole moieties. The observed dichroism thus represents an
Experimental
average over the different orientations. For 2+–CS2 we observe
−
General considerations
a weaker difference peak with positive polarity for the π* imida-
zole resonance. This indicates an upright, but tilted orientation
of the imidazole unit with respect to the surface.
All preparations involving air-sensitive compounds were carried
out under an atmosphere of dry nitrogen by using standard
Schlenk techniques or in a conventional argon-filled glove box.
Solvents and reagents were appropriately dried and purified by
conventional methods and stored under inert gas atmosphere.
The N-heterocyclic carbenes 111b and 211a and the azolium salts
[3H]Cl12 and [4H]Br13 were prepared by slight variation of pub-
lished procedures. Elemental analyses were carried out by the
microanalytical laboratory of the Institute of Thermal Energy
Management at the University of Kassel. NMR spectra were
recorded with the following Varian spectrometers: VNMRS-500
(500 MHz) and Varian 400-MR (400 MHz). Chemical shifts (δ)
are given in ppm and are referenced to the signals due to the
A quantitative analysis of the C K-edge NEXAFS spectra was
performed to determine the average molecular tilt angles. The
orientation of aromatic units with respect to the surface normal
were determined using the π* transitions. The intensities of these
resonances as a function of the X-ray incidence angle Θ are eval-
uated using published procedures for a vector-type orbital.29
This analysis yields an average tilt angle for the aromatic ring
planes versus the surface normal of 53° 5° for 1+–CS2−. This
value represents an average over the phenyl and imidazole orien-
tation and the actual tilt angle of the phenyl rings is most likely
−
higher. For the SAMs fabricated from 2+–CS2 we found an
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 2986–2994 | 2991

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residual protio impurities of the solvents used relative to tetra-
methylsilane for 1H and to the respective solvent signal for
13C. IR spectra were obtained with a Bruker Alpha-T FT-IR
spectrometer (KBr pellets). Raman spectra were obtained at a
temperature of 10 K with a Bruker IFS 66/CS FT-NIR spec-
trometer equipped with a FRA 106 FT-Raman module. An
Adlas Nd:YAG laser (350 mW, wavelength 1064 nm) was used
as a light source. Optical spectra were obtained with a Perkin
Elmer Lambda 40 UV/Vis spectrometer. Mass spectra were
obtained with a Bruker Esquire 3000 spectrometer (ESI) and a
quadrupole ion-trap spectrometer (ESI and APCI) Finnigan
LCQDECA (ThermoQuest, San José, USA).
for [C18H16N2S2Na]+. Calc. for C18H16N2S2 (324.6): C, 66.60;
H, 4.94; N, 8.63. Found: C, 66.39; H, 4.94; N, 8.63%.
1,3-Diethylbenzimidazolium-2-dithiocarboxylate (4+–CS2−).
Potassium-tert-butoxide (146 mg, 1.3 mmol) was added to a sol-
ution of [4H]Br (281 mg, 1.1 mmol) in THF (25 mL) affording
a yellow precipitate. Subsequently, carbon disulfide (120 μL,
152 mg, 2.0 mmol) was added and the mixture stirred for
15 min. Volatile components were removed in vacuo. The crude
product was purified by column chromatography (silica gel,
dichloromethane), affording a dark red microcrystalline solid.
1
Yield 121 mg (44%), mp 211–212 °C (dec.). H NMR (CDCl3):
δ 1.59 (t, J = 7.3 Hz, 6H, CH3), 4.43 (q, J = 7.3 Hz, 4H, CH2),
7.56 (m, 4H, CH). 13C NMR (CDCl3): δ 14.3, 40.95, 112.4 (2C,
5,8-CH), 126.05 (2C, 6,7-CH), 129.90 (2C, 4,9-C), 152.59 (1C,
NCN), 224.01 (1C, CS2). IR (KBr): ν 2960 (w), 1501 (s), 1467
(s), 1435 (s), 1380 (m), 1359 (m), 1340 (m), 1138 (w), 1095
(m), 1056 (s), 1034 (m), 1021 (m), 924 (m), 807 (m), 745 (s).
UV-vis: λ(ε) 229 (12800), 273 (8150), 309 (10850), 362
(10600), 438 (150), 528 (150). HRMS/ESI(+): m/z 273.0497
[M + Na]+, 273.0491 calc. for [C12H14N2S2Na]+. Calc. for
C12H14N2S2 (250.5): C, 57.53; H, 5.63; N, 11.27. Found: C,
57.27; H, 5.87; N, 10.69%.
Preparative work
1,3-Bis(2,6-diisoproylphenyl)imidazolium-2-dithiocarboxylate
(1+–CS2−). Carbon disulfide (0.30 mL, 0.38 g, 5.0 mmol) was
added to a solution of 1 (1.75 g, 4.5 mmol) in THF (30 mL).
The solution was stirred for 15 min. Volatile components were
removed in vacuo, affording the product as a red microcrystalline
solid, which turned out to be analytically pure. Yield 2.06 g
(98%). Spectroscopic data were essentially identical to those
reported by Delaude et al.4
(2+–CS2−).
X-Ray crystal structure determinations
1,3-Diisopropylimidazolium-2-dithiocarboxylate
Carbon disulfide (0.36 mL, 0.46 g, 6.0 mmol) was added to a
solution of 2 (614 mg, 4.0 mmol) in THF (25 mL). The solution
was stirred for 15 min. Volatile components were removed in
vacuo. The crude product was purified by column chromato-
graphy (silica gel, dichloromethane–ethyl acetate 5 : 1), affording
For each data collection a single-crystal was mounted on a glass
fibre and all geometric and intensity data were taken from this
sample. Data collection using Mo-Kα radiation (λ = 0.71073 Å)
was made on a Stoe IPDS2 diffractometer equipped with a 2-
circle goniometer and an area detector. Absorption correction
was done by integration using X-red.37 The data sets were cor-
rected for Lorentz and polarisation effects. The structures were
solved by direct methods (SHELXS97) and refined using alter-
nating cycles of least squares refinements against F2
(SHELXL97).38 All non H atoms were found in difference
Fourier maps and were refined with anisotropic displacement
parameters. H atoms were placed in constrained positions
according to the riding model with the 1.2 fold isotropic displa-
cement parameters. Crystallographic details are collected in
Table 4. Graphical representations were made using ORTEP-3
win.39
a
red microcrystalline solid. Yield 404 mg (76%), mp
1
232–234 °C. H NMR: δ 1.51 (d, J = 6.7 Hz, 12H, CH(CH3)2),
4.96 (sept, J = 6.7 Hz, 2H, CH(CH3)2), 6.99 (s, 2H, = CHN).
13C NMR: δ 22.7 50.3, 114.5, 149.0, 225.7. IR (KBr): ν 3094
(s), 2976 (s), 2931 (m), 2874 (w), 1565 (m), 1479 (s), 1463 (s),
1368 (m), 1211 (s), 1166 (m), 1071 (m), 1047 (s), 753 (w), 716
(s). UV-vis: λ(ε) 230 (9500), 266 (7100), 359 (13700), 433
(150), 526 (100). HRMS/ESI(+): m/z 229.0826 [M+H]+,
229.0828 calc. for [C10H16N2S2H]+. Calc. for C10H16N2S2
(228.5): C, 52.56; H, 7.06; N, 12.32. Found: C, 52.18; H, 7.10;
N, 12.12%.
1,3-Dibenzylimidazolium-2-dithiocarboxylate (3+–CS2−). [3H]
Cl (650 mg, 2.3 mmol) was stirred for 20 min with 4 Å molecu-
lar sieves (ca. 2 g) in acetonitrile (25 mL). Potassium-tert-butox-
ide (300 mg, 2.7 mmol) was added and the solution stirred for a
further 15 min. Carbon disulfide (0.45 mL, 0.57 g, 7.5 mmol)
was added and the solution stirred for a further 15 min. Volatile
components were removed in vacuo. The crude product was
purified by column chromatography (silica gel, dichloro-
methane–ethyl acetate 5 : 1), affording a dark red microcrystal-
line solid. Yield 230 mg (31%), mp 178–179 °C (dec.). 1H
NMR (CDCl3): δ 5.29 (s, 4H, CH2) 6.58 (s, 2H, = CHN), 7.40
(m, 10H, Ph). 13C NMR (CDCl3): δ 51.7, 117.5, 129.2, 129.25,
129.3, 132.8, 150.3, 223.9. IR (KBr): ν 3162 (w), 3138 (w),
1567 (m), 1496 (s), 1452 (s), 1442 (s), 1360 (m), 1247 (s), 1164
(m), 1056 (s), 966 (w), 749 (m), 730 (m), 711 (s), 691 (m). UV-
vis: λ(ε) 238 (9000), 256 (10000), 360 (13800), 444 (250), 531
(150). HRMS/ESI(+): m/z 347.0652 [M + Na]+, 347.0647 calc.
Optical second harmonic generation (SHG)
Gold substrates were prepared at the Institute of Nanostructure
Technology and Analytics, Kassel University, Germany (200 nm
gold with a 15 nm titanium interlayer for adhesion promotion
evaporated on an Si(100) wafer). For the SHG measurements the
gold substrates were cut to pieces of 1 × 1 cm2 each and placed
in a homemade cuvette, whose quartz glass window was sealed
with a Chemraz O-ring. The cuvette was filled with pure
dichloromethane at the beginning of the experiments and the
pure solvent was subsequently replaced by the solution of the
adsorbate species. The adsorption process was measured in situ
and in real time by SHG by using a ns-pulsed Nd:YAG laser
(GCR-170, Spectra Physics) at a repetition rate of 10 Hz with a
fundamental wavelength of 1064 nm. The laser beam was inci-
dent on the gold surface under an angle of 45° with a fluence of
2992 | Dalton Trans., 2012, 41, 2986–2994
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 4 Crystal data and structure refinement details
−
−
2+–CS2
3+–CS2
4+–CS2− · CHCl3
Empirical formula
Molecular weight
Crystal size/mm
C10H16N2S2
228.37
0.60 × 0.45 × 0.03
0.85/0.98
C18H16N2S2
324.45
0.45 × 0.34 × 0.09
0.86/0.97
193(2)
C13H15Cl3N2S2
369.74
0.19 × 0.15 × 0.02
0.88/0.98
218(2)
T
min/Tmax
T/K
173(2)
Crystal system
Space group
a/Å
b/Å
c/Å
Tetragonal
P 41 21 2
9.2048(4)
9.2048(4)
28.6900(17)
Monoclinic
P 21/c
Monoclinic
P 21/n
11.5106(15)
18.013(2)
16.037(2)
91.115(10)
3324.5(7)
8
9.1788(10)
14.2940(13)
13.2075(16)
94.883(9)
1726.6(3)
4
β (°)
V/Å3
2430.9(2)
8
1.248
Z
Dc/g cm−1
1.296
0.318
1.422
0.764
μ/mm−1
0.404
θ range/°
Refl. measured
Unique refl.
Rint
2.32 to 24.99
5036
1230
0.0707
1148
0.0338, 0.0853
0.0360, 0.0862
−0.209/0.245
1.70 to 25.36
21 202
5974
0.1497
3083
0.0812, 0.2047
0.1338, 0.2333
−0.530/0.369
2.10 to 25.00
7557
3028
0.0337
1980
0.0717, 0.1864
0.1038, 0.2048
−0.792/0.894
Refl. observed
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
Δρmin max
//e Å−3
/
20 mJ cm−2. After generation of the second harmonic light, light
of the fundamental wavelength was filtered out by using several
colour glasses (BG39, Schott) and a monochromator. Finally, the
second harmonic signal was detected by a photomultiplier tube
and processed with a boxcar for storage on a PC. A fraction of
the fundamental laser light was benched off by a dielectric beam
splitter. This light was aligned onto a y-cut quartz plate in order
to generate a reference SHG signal. It was detected comparable
to the signal of the sample and acted to normalise the SHG
signal from the sample. Thus, intensity fluctuations of the laser
light were minimised. Details of the experimental setup for the
investigation of the adsorption and monolayer formation have
been published elsewhere.17
Near-edge X-ray absorption fine structure spectroscopy
NEXAFS spectra were recorded at the National Synchrotron
Light Source (NSLS) U7A beamline at Brookhaven National
Laboratory, using an elliptically polarised beam with ∼85%
p-polarisation. This beam line uses a monochromator and 600
l mm−1 grating that provides a full-width at half-maximum
(FWHM) resolution of ∼0.15 eV at the carbon K-edge (285 eV).
The monochromator energy scale was calibrated using the
285.35 eV C 1 s → π* transition on a graphite transmission grid
placed in the path of the X-rays. C K-edge spectra were normal-
ised by the spectrum of a clean gold surface prepared by evapor-
ation of gold in vacuo. Both reference and signal were divided
by the NEXAFS signal of an upstream gold-coated reference
mesh to account for beam intensity variations.29 Partial electron
yield was monitored with a channeltron detector with the bias
voltage maintained at −150 V for C K-edge. The samples were
mounted to allow rotation about the vertical axis to change the
angle between the sample surface and the incident X-ray beam.
The NEXAFS angle is defined as the angle between the incident
X-ray beam and the sample surface.
X-Ray photoelectron spectroscopy
All XP spectra were collected on a Kratos AXIS Ultra DLD
instrument (Kratos, Manchester, England) equipped with a
monochromatic Al Kα X-ray source (photon energy = 1486.6
eV). The photoelectron take-off angle was normal to the sub-
strate while the photoelectron binding energy scale was cali-
brated to the Au 4f7/2 emission (84.0 eV) of the underlying gold
substrate. The reported XPS compositions are an average from
three spots per sample and calculated from peak areas taken
from both a survey scan (0 to 1100 eV for C 1 s and Au 4f) and
selected region scans (524–544 eV for O 1s; 390–410 eV for N
1s; 155–173 eV for S 2p) acquired at an analyser pass energy of
80 eV. Molecular environments of the samples were character-
ised by high-resolution (analyser pass energy = 20 eV) spectra
from the S 2p, N 1 s, and C 1 s regions. For all peak quantifi-
cations a linear background was subtracted. Peak areas were nor-
malised by the manufacturer-supplied sensitivity factors and
surface concentrations were calculated using Casa XPS software.
Acknowledgements
This work was funded in part by NESAC-BIO (NIH grant
EB-002027). T. W. and J. E. B. thank David G. Castner for
support and Daniel Fischer and Cherno Jaye (NIST) for provid-
ing them with the experimental equipment for NEXAFS spec-
troscopy and their help at the synchrotron. NEXAFS studies
were performed at the NSLS, Brookhaven National Laboratory,
which is supported by the U. S. Department of Energy, Division
of Materials Science and Division of Chemical Sciences. H. M.
thanks Ms U. Cornelissen for recording the Raman spectra.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 2986–2994 | 2993

View Article Online
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2994 | Dalton Trans., 2012, 41, 2986–2994
This journal is © The Royal Society of Chemistry 2012