Self-assembled monolayers based on Pd-containing organometallic thiols: Preparation and structural characterization

Article abstract of DOI:10.1021/jp904865k

Multilayers and self-assembled monolayers of on-purpose-prepared organometallic thiolates, trans[Pd(PBu₃)₂(SCOCH ₃)₂], trans-[(C₆H₅C≡C) Pd(PBu₃)₂(SCOCH₃)],

Full text of DOI:10.1021/jp904865k

14730
J. Phys. Chem. A 2009, 113, 14730–14740
Self-Assembled Monolayers Based on Pd-Containing Organometallic Thiols: Preparation
and Structural Characterization†
Rosa Vitaliano,† Ilaria Fratoddi,† Iole Venditti,† Giuseppina Roviello,‡ Chiara Battocchio,*,§
Giovanni Polzonetti,§ and Maria Vittoria Russo†
Department of Chemistry, UniVersity of Rome “La Sapienza” P. le A. Moro, 5- 00185, Rome, Italy,
Department of Chemistry, UniVersity of Napoli “Federico II”, Complesso di Monte Sant’Angelo, Via Cintia,
80126 Napoli, Italy, Department of Physics, INSTM, CNISM and CISDiC, UniVersity of Rome “Roma Tre” Via
della Vasca NaVale 84 00146 Rome, Italy
ReceiVed: May 25, 2009; ReVised Manuscript ReceiVed: July 16, 2009
Multilayers and self-assembled monolayers of on-purpose-prepared organometallic thiolates, trans-
[Pd(PBu3)2(SCOCH3)2], trans-[(C6H5CtC)Pd(PBu3)2(SCOCH3)], and trans,trans-[(CH3COS)Pd(PBu3)2-
(CtC-C6H4-C6H4-CtC)(PBu3)2Pd(SCOCH3)] were deposited onto gold surfaces. High-resolution X-ray
photoelectron spectroscopy and near-edge X-ray absorption fine structure measurements allowed us to assess
the anchoring of the organometallic thiols onto gold substrates; the interaction occurring at the interface; and
their molecular orientation on the surface with tilt angles of about 30°-40°, depending on the investigated
molecule. The molecule packing density/coverage was also assessed.
1. Introduction
monolayers on gold substrates.13 Among others, organometallic
oligomers and polymers have recently received great interest
due to the possibility of coupling the chemical, electronic,
optical, and redox properties of transition metal complexes to
those of organic spacers.14-19 These materials show useful
properties for applications in modern technology,20 and molec-
ular models of long-chain organometallic polymers allow in-
depth understanding and modulation of the molecular structure
and electronic properties of the corresponding polymers.21
Moreover, if the oligomers possess a thiol group at both ends
as an anchoring unit, the bridging of two surfaces/electrodes is
possible,22 and linearly structured π-conjugated organic and
inorganic building blocks can be used as connecting units. The
properties of such molecules strongly depend on the nature of
the redox moiety and the appropriate connecting spacer.23
Organic thiols have received a great deal of attention,24 and the
use of redox-active organometallic complexes offers perspectives
for the design and preparation of self-assembled monolayers.25
For example, the fabrication and characterization of fully
organometallic multilayer thin films composed of poly(ferro-
cenylsilane) polyanions and polycations using layer-by-layer
self-assembly was recently reported.26 The driving force for this
research is the expectation of fast computational systems through
the miniaturization of microelectronic circuit components to the
nanoscale.27-30 The study of structure/property relationships in
wirelike molecules is a key topic for current investigations, and
the use of the molecular structure in electronic circuits is usually
achieved via terminal thiol-gold contacts.31 The efficiency of
electronic communication is based on the use of molecules that
possess delocalized π-systems with low HOMO-LUMO gaps.
Rigid-rod π-conjugated oligomers and model molecules con-
stitute a potentially new class of molecular wires.32 There have
been several studies of electron transfer involving such materials
with thiol, R,ω-dithiol, thioacetyl, or R,ω-dithioacetyl end
groups,33,34 which are necessary for the formation of self-
assembled monolayers or covalent connections between proxi-
mate gold electrodes.35,36
In recent years, scientific research has been devoted to the
emerging field of the preparation of nanostructured materials
for applications ranging from nanomedicine to nanoelectronics,1
and the formation and control of self-assembled monolayers
(SAMs) of organic thiols on several substrates, such as noble
metals, have attracted interest in the research community.2,3
Several efforts devoted to in-depth theoretical understanding
of the geometric and electronic structure of self-assembled thiols
have been developed to provide information on properties of
SAMs4 in view of potential applications of these systems.5 SAM
structures are governed by several factors, among them the
nature of the substrate and adsorbate, the interaction between
the molecules and the substrate, and the intramolecular interac-
tions. SAM properties are determined by a combination of the
molecular structure and the SAM organization. A number of
studies have been reported for aromatic thiol SAMs on gold.6
The molecular orientation and packing density strongly depends
on the interplay between the sulfur-gold and the intermolecular
lateral interactions between the SAM constituents; in this
context, several works report that the packing of the organic
groups can be improved by stacking interactions between the
aromatic rings in the adjacent adsorbates, thus increasing the
packing density.7 In this regard, oligo(aryleneethynylene)s
represent an attractive class of molecules8,9 because of their
rigid-rod structure, and although they show a low barrier to
rotation along the aryl-ethynyl bond,10,11 some of these molecules
with terminal thiol groups have been integrated into electronic
circuits.12
The synthesis of new oligo(aryleneethynylene) molecular
wires of ∼4 nm length scale with terminal thiolates has been
recently reported and used for the preparation of self-assembled
† Part of the “Vincenzo Aquilanti Festschrift”.
* Corresponding author. Phone: (+) 39-06-57333388. Fax: (+) 39-06-
57333390. E-mail: battocchio@fis.uniroma3.it.
† University of Rome “La Sapienza”.
‡ University of Napoli “Federico II”.
§ University of Rome “Roma Tre”.
10.1021/jp904865k  2009 American Chemical Society
Published on Web 08/12/2009

SAMs Based on Pd-Containing Organometallic Thiols
J. Phys. Chem. A, Vol. 113, No. 52, 2009 14731
SCHEME 1: Chemical Structures of Pd(II) Chlorides, Thioacetate Complexes, Free and Gold Anchored Thiols
In this framework, we have devoted our research to the
synthesis and characterization of mono and bimetallic palladium
complexes in which the metal atom is σ-bound to sulfur-
containing ligands. In this paper, we describe the preparation
and characterization of SAMs based on Pd(II) organometallic
complexes grown onto a Au/Si(111) surface, whose structure
is shown in Scheme 1. The use of organometallic thiol
complexes as stabilizing agents for gold surfaces or nanoclusters
offers a promising perspective for the applications of these
materials,37,38 and in-depth investigation of the molecular
structures of organometallic SAMs is an important goal for
defining their self-assembling behavior and properties. High-
resolution X-ray photoelectron spectroscopy (HR-XPS) is a very
well suited technique to investigate the molecule-substrate
interaction in SAMs because the high spectral resolution and
photon flux allow one to ascertain the identity of the
headgroup-substrate interface. HR-XPS measurements com-
bined with near edge X-ray absorption fine structure (NEXAFS)
and reflection absorption infrared spectra (IRRAS) studies were
carried out to achieve a structural and electronic characterization
of the organometallic SAMs.
and 31P NMR spectra were recorded on a Varian 300 spectrom-
eter at 300, 75, and 121 MHz, respectively. 1H and 13C chemical
shifts (parts per million) are reported in δ values relative to
internal standard (CH3)4Si; CDCl3 (δ 7.24) and C6D6 (δ 7.15)
1
were used as solvents for H NMR spectra as well as for 13C
NMR spectra (CDCl3 δ 77.0; C6D6 δ 128.0 ppm). The 31P
chemical shifts are relative to a H3PO4 (85%) probe. UV-vis
spectra were recorded by a Varian-Cary 100 spectrophotometer.
Photoluminescence (PL) spectra were registered on a Perkin-
Elmer LS 50 spectrofluorometer. UV and PL measurements
were performed at room temperature using CH2Cl2 quantitative
solutions of the compounds in quartz cells. The molar extinction
coefficient, ε°, and the relative quantum yield, η, of CH2Cl2
solutions of the complexes were also determined as reported in
the literature.39 Elemental analyses were provided by the
Servizio di Microanalisi of the Department of Chemistry, on
an EA 1110 CHNS-O instrument.
XPS spectra were obtained using a custom-designed spec-
trometer. A non-monochromatized Mg KR X-ray source (1253.6
eV) was used, and the pressure in the instrument was maintained
at 1 × 10-9 Torr throughout the analysis. The experimental
apparatus consists of an analysis chamber and a preparation
chamber separated by a gate valve. An electrostatic hemispheri-
cal analyzer (radius 150 mm) operating in the fixed analyzer
transmission mode and a 16-channel detector giving a total
instrumental resolution of 1.0 eV as measured at the Ag 3d5/2
2. Experimental Section
FTIR spectra in the range 400-4000 cm-1 were recorded
with a Bruker Vertex 70 Fourier transform spectrometer as film
1
deposited from CH2Cl2 solutions on ZSM5 CsI cells. H, 13C,

14732 J. Phys. Chem. A, Vol. 113, No. 52, 2009
Vitaliano et al.
core level were used. The film samples were prepared by
dissolving our materials in CHCl3 and spinning the solutions
onto polished stainless steel substrates. The samples showed
good stability during the XPS analysis, preserving the same
spectral features and chemical composition. The spectra were
energy-referenced to the C1s signal of aliphatic C atoms, having
a binding energy of BE ) 285.00 eV, in those samples
containing mainly aliphatic carbons, to the C 1s signal of BE
) 284.7 eV in those containing more aromatic carbon atoms,
in agreement with literature data.40 Curve-fitting analysis of the
C 1s, Pd 3d, P 2p, S 2p, and O 1s spectra was performed using
Gaussian curves as fitting functions (typical full width at half-
maximum, fwhm, in the range 1.7-2.1 eV) after subtraction of
a Shirley-type background. Quantitative evaluation of the atomic
ratios was obtained by analysis of the XPS signal intensity
employing Scofield’s atomic cross-section values and experi-
mentally determined sensitivity factors.41
synchrotron radiation was varied from normal (90°) to grazing
(20°) incidence with respect to the sample surface. The photon
energy and resolution were calibrated and experimentally tested
at the K absorption edges of Ar, N2, and Ne. In addition, our
carbon K-edge spectra have been further calibrated using the
resonance at 285.50 eV assigned to the C 1s-π* ring transi-
tion. The raw C K-edge NEXAFS spectra were normalized to
the incident photon flux by dividing the sample spectrum by
the spectrum collected on a freshly sputtered gold surface. The
spectra were then normalized, subtracting a straight line that
fits the part of the spectrum below the edge and assessing to 1
the value at 320.00 eV. IRRAS measurements were carried out
on a Bruker Vertex 40 instrument equipped with a grazing
incidence reflection unit (Bruker). Incidence light was reflected
from the samples at angles of incidence 0° and 90°. The final
spectra were averaged from 64 scans at a spectral resolution of
4 cm-1.
2.1. Materials. All reactions were performed under inert
argon atmosphere. Solvents (Carlo Erba) were dried on Na2SO4
and subjected to argon bubbling before use. All chemicals,
unless otherwise stated, were obtained from commercial sources
and used as received. The precursor palladium complex, trans-
[Pd(PBu3)2Cl2], was prepared according to literature methods.47
4,4′-Diethynylbiphenyl, DEBP, was obtained starting from the
silyl derivative, 4,4′-trimethylsilyldiethynylbiphenyl, DEBP-Si,
following a reported procedure.48 Phenylacetylene was pur-
chased from Aldrich and distilled before use. Potassium
thioacetate and sodium hydrosulfide hydrate were purchased
from Aldrich and were used without further purifications.
Preparative thin-layer chromatography separation was performed
on 0.7 mm silica plates (Merck Kieselgel 60 GF254), and
chromatographic separations were obtained with 70-230 mesh
silica (Merck) and proper solvents as the eluant.
2.2. Syntheses. The full synthesis methodology and charac-
terization of precursor complexes trans-[(C6H5-CtC)Pd-
(PBu3)2Cl] (1) and trans,trans-[Cl(PBu3)2Pd(CtC-C6H4-
C6H4-CtC)Pd(PBu3)2Cl] (2), of thiolates trans-[Pd(PBu3)2-
(SCOCH3)2] (3) and trans-[(C6H5CtC)Pd(PBu3)2(SCOCH3)]
(4), and of thiols trans-[Pd(PBu3)2(SH)2] (6) and trans-
[(C6H5CtC)Pd(PBu3)2(SH)] (7) are reported in the Supporting
Information. The synthesis and characterization of the complexes
trans,trans-[(CH3COS)Pd(PBu3)2(CtC-C6H4-C6H4-CtC)-
(PBu3)2Pd(SCOCH3)] (5) and trans,trans-[(HS)Pd(PBu3)2-
(CtC-C6H4-C6H4-CtC)(PBu3)2Pd(SH)] (8) are here de-
scribed as representative examples.
Data collections for X-ray diffraction (XRD) crystal structure
determination were performed in flowing N2 at 173 K for 2
and at room temperature for 5 and 7 on a Bruker-Nonius kappa
CCD diffractometer (Mo KR radiation, CCD rotation images,
thick slices, ꢀ scans + ω scans to fill the asymmetric unit).
Cell parameters were determined from 266 reflections in the
range 4.31° e θ e 18.816°, 99 reflections in the range 3.86° e
θ e 21.45°, and 16 reflections in the range 3.62° e θ e 14.87°
for 2, 5, and 7, respectively. Semiempirical absorption correc-
tions (multiscan SADABS)42 were applied. All structures were
solved by direct methods (SIR 97 package)43 and refined by
the full matrix least-squares method (SHELXL program of the
SHELX97 package)44 on F2 against all independent measured
reflections using anisotropic thermal parameters for all non-
hydrogen atoms. H atoms were placed in calculated positions
with Ueq equal to those of the carrier atom and refined by the
riding method. Evidence of high thermal motion and positional
disorder was found for the butyl groups in 2 and 7.
HR-XPS measurements were performed at the synchrotron
storage ring ELETTRA using the BACH (beamline for ad-
vanced dichroism) beamline and relative experimental station
that is connected to an undulator front end. To probe the S 2p
and Au 4f core levels, an excitation energy of 699 eV was used,
a photon energy value leading to the optimization of the
photoelectron yield for both core levels. The spectra were
acquired in normal emission geometry, and the resolution was
better than 0.1 eV. The S 2p binding energy for every sample
was individually calibrated using th eAu 4f7/2 photoemission
line at 83.95 eV45 of the thiol-covered Au substrate. The spectra
were fitted by Gaussian functions, and a Shirley background
was previously subtracted.46 The S 2p3/2, S 2p1/2 spin-orbit
doublet was fitted by using the same full width at half-maximum
(fwhm) value for both components, a spin-orbit splitting of
1.20 eV, and an intensity ratio S 2p3/2/S 2p1/2 of 2/1. When
several different species were found in the same spectrum, the
same fwhm value was used for all individual peaks.
Trans,trans-[(CH3COS)Pd(PBu3)2(C′C-C6H4-C6H4-C′C)-
(PBu3)2Pd(SCOCH3)] (5). Compound trans,trans-[Cl(PBu3)2-
Pd(CtC-C6H4-C6H4-CtC)Pd(PBu3)2Cl] (2), (0.100 g, 0.08
mmol) was dissolved in 50 mL of CH2Cl2, and 0.019 g (0.17
mmol) of KSCOCH3 was added. The solution was stirred at
room temperature for 4 days, then filtered, and the solvent was
removed. The crude product (5) was an orange-brown solid that
was crystallized from CH2Cl2/CH3OH at 4 °C. Pale-yellow
single crystals were obtained. Yield: 95%. Melting point: 62-63
°C. Anal. calcd: C 59.53 (59.50), H 9.11 (8.96), S 5.04 (4.67).
FTIR (film, cm-1): 2108 (ν CtC), 1623 (ν CdO), 1261 (ν
S-CdO). UV (CH2Cl2, nm): λmax ) 332; ε° ) 64.78 (103 L
mol cm-1). Luminescence emission (CH2Cl2, nm): λmax ) 450;
Angular dependent synchrotron-induced NEXAFS experi-
ments were performed at the ELETTRA storage ring at the
bending magnet for emission absorption and reflectivity beam-
line, installed at the left exit of the 8.1 bending magnet exit.
The apparatus is based on a bending magnet as a source,
beamline optics delivering photons from 5 up to about 1600
eV with a selectable degree of ellipticity. The UHV end station
has a movable hemispherical electron analyzer and a set of
photodiodes to collect angle-resolved photoemission spectra,
optical reflectivity, and fluorescence yield. To examine the
molecular orientation, the incident angle of the linearly polarized
1
η ) 0.8%. NMR (CDCl3, δ ppm): H, 0.92 (t, J ) 7.2 Hz,
CH3), 1.44 (m, CH2), 1.55 (m, CH2), 1.94 (m, P-CH2), 2.36
(s, CH3-CdO), 7.30 (d, Ar-H), 7.45 (d, Ar-H); 13C, 13.74
(s, CH3), 24.39 (t, JC-P ) 6.7 Hz, CH2), 23.49 (t, JC-P ) 13.7
Hz, P-CH2), 26.45 (s, CH2), 35.03 (s, CH3-CdO), 106.21 (t,
JC-P ) 17.3 Hz, CtC), 107.54 (t, JC-P ) 4.5 Hz, CtC),

SAMs Based on Pd-Containing Organometallic Thiols
J. Phys. Chem. A, Vol. 113, No. 52, 2009 14733
126.25 (s, Ar-C), 126.91 (s, Ar-C), 130.81 (s, Ar-C), 137.58
(s, Ar-C), 205.25 (s, CdO); 31P (CDCl3, δ ppm): 10.40 (s).
Trans,trans-[(HS)Pd(PBu3)2(CtC-C6H4-C6H4-C′C)-
(PBu3)2Pd(SH)] (8). Compound trans,trans-[Cl(PBu3)2Pd-
(CtC-C6H4-C6H4-CtC)Pd(PBu3)2Cl] (2) (0.10 g, 0.08
mmol) was dissolved in 20 mL of THF, and 0.037 g (0.66
mmol) of NaSH was added. The solution was stirred at room
temperature for 2 days, and the subsequent work-up followed
the same procedure reported in compound 6. The pure product
is a yellow, waxy solid. Yield: 87%. Melting point: 90 °C. Elem.
anal. % (calcd %): C 60.04 (59.66); H 9.36 (9.23); S 5.35 (4.98).
FTIR (film, cm-1): 2108 (ν CtC); 1603 (ν CdC); 2570 (ν
S-H). UV (CH2Cl2, nm): λmax ) 322; ε° ) 64.48 (103 L mol
cm-1). Luminescence emission (CH2Cl2, nm): λmax ) 354, η )
0.84%. NMR (CDCl3, δ ppm): 1H, 0.93 (t, J ) 7.0, CH3), 1.45
(m, CH2), 1.57 (m, CH2), 1.95 (m, P-CH2), 13.32 (t, J ) 11.4
Hz, S-H), 7.30 (d, Ar-H), 7.46 (d, Ar-H); 13C, 13.78(s, CH3),
Figure 1. ORTEP view of 2. Thermal ellipsoids are shown at 30%
probability level. Butylic tails of PBu3 and all hydrogen atoms are not
shown for clarity. i ) -x + 1, -y - 1, -z + 1.
Section). The chemical structures of palladium chlorides,
thioacetates, and thiols anchored on gold are depicted in
Scheme 1.
Metal-carbon coupling between a Pd(II) dichloride complex
and terminal alkynes has been applied for the preparation of
complexes 1 and 2 with relatively high yields, following a
dehydrohalogenation general synthetic pathway.49 To avoid the
isolation of the oxidatively unstable free thiol complexes,
potassium thioacetate was selected as the nuclophile for
displacing the chloride ligands. The target compounds 3, 4, and
5 were synthesized by a typical metal-halide-exchange reaction
according to a procedure similar to that described for Pd(II)
complexes by different groups.50-57
26.56 (s, CH2), 24.38 (t, JC-P ) 6.6 Hz, CH2), 23.53 (t, JC-P
)
13.9 Hz, P-CH2), 105.45 (t, JC-P ) 4.5 Hz, CtC), 109.04 (t,
J ) 17.7 Hz, CtC), 132.53 (s, Ar-C), 130.99 (s, Ar-C),
128.00 (s, Ar-C), 126.58 (s, Ar-C); 31P (CDCl3, δ ppm):
9.99 (s).
2.3. Multilayers and SAMs Preparation. For the prepara-
tion of multilayers and self-assembled monolayers, terminal thiol
compounds were in situ obtained by a deacylation procedure
carried out on precursor thiolate complexes 3, 4, and 5, and
allowed to self-assemble on gold surfaces. Gold-coated silica
wafers prepared by growing a Au film 4000 Å thick onto Si(111)
substrates were cut into slides (ca. 1 cm2) and washed with
several organic solventssacetone, ethanol, chloroformsand
blown dry with nitrogen. The substrates were then dipped into
the proper, just obtained thiol solution to achieve the anchoring.
The FTIR spectra of the Pd(II) acetylyde complexes 1, 2, 4,
5, 7, and 8 showed a single signal of the CtC stretching mode
around 2100 cm-1, and the presence of the thioacetyl ligand
was assessed by the sharp signal at around 1626 cm-1 (νCdO)
and 946 cm-1 (νC-S) in compounds 3, 4, and 5. Such
absorption bands evidence a unidentate coordination of thio-
acetyl via the sulfur atom to palladium and indicate the absence
of significant palladium-oxygen interactions,58 confirmed by
the crystal structure (see, for example, Figure 1). The IR spectra
of thiol-containing compounds 6, 7, and 8 showed a weak signal
at about 2500 cm-1, consistent with the S-H stretching
mode.59,60
1H NMR spectra of the compounds exhibited the typical
pattern of the phosphine resonances at about 0.9, 1.4, 1.6, and
1.9 ppm and a double doublet typical pattern of the biphenyl
or phenyl ligand at 7.29 and 7.45 ppm. A single signal at around
2.36 ppm was detected for compounds 3, 4, and 5, confirming
the presence of the thioacetyl ligand. The signal at about 13.3
ppm in compounds 6, 7, and 8 confirmed the presence of S-H
terminal thiol ligands.
The 13C NMR spectra show the expected resonances of
phosphine ligands, diethynylbiphenyl moiety, and of ending
phenyl ligands for all the examined complexes. In addition, a
single signal at about 205 ppm due to the carbonyl carbon
appeared in the spectra of thioacetate complexes 3, 4, and 5.
Single resonances in the 31P NMR spectra were observed for
all compounds at about 10.44 ppm for chlorides, in the range
9.43-10.40 ppm for thioacetates, and in the range 8.45-9.99
ppm for thiol Pd(II)-based complexes.
XRD measurements highlight the molecular structure of 2,
reported in Figure 1. The asymmetric unit contains a half
molecule of the dimer, laying the molecule on an inversion
center located at the midpoint of the central C-C bond of the
biphenyl group, showing analogies with related compounds.61
The coordination environment of the palladium atom is square-
planar (angles are in the range 84.6-94.4°), with two trans PBu3
groups and trans chloride and acetylide moieties. As already
noted in the literature,62 a slight deformation in the coordination
of the metal atom is observed, probably due to a repulsive
interaction between the chloride atom and the phosphine ligands.
The Pd-C and CtC bond lengths of 1.934(12) and 1.213(15)
Å, respectively, are typical for metal-σ-acetylide moieties.
Preparation and anchoring of multilayer M(3): compound 3
(0.040 g, 0.06 mmol) was dissolved in 20 mL of THF, and 260
µL of NH4OH (30%) was added.
Preparation and anchoring of multilayer M(4): compound 4
(0.030 g, 0.04 mmol) was dissolved in 25 mL of THF, and 110
µL of NH4OH was added.
Preparation and anchoring of multilayer M(5): compound 5
(0.035 g, 0.02 mmol) was dissolved in 15 mL of THF, and 125
µL of NH4OH was added.
The solutions were stirred at 30 °C for 2 h and filtered on
Celite, and freshly washed gold substrates were dipped into the
solution for 4 h. The multilayers M(3), M(4), and M(5) were
rinsed with different solvents (ethanol, THF, and acetone) to
achieve the formation of SAM(3), SAM(4), and SAM(5) films
in the monolayer thickness regime.
3. Results and Discussion
3.1. Preparation and Characterization of Organometallic
Palladium Complexes. trans-[Pd(PBu3)2Cl2] reacted with phe-
nylacetylene or 4,4′-diethynylbiphenyl to give mononuclear and
binuclear complexes 1 and 2, respectively, and these compounds
were used as starting materials for the preparation of thioacetate
complexes 3, 4, and 5, suitable for the self-assembling procedure
on gold surfaces. In our study, the deacylation of thioacetyl
groups to achieve terminal thiols was carried out in situ by using
the weak base NH4OH, and a series of well-defined, easily
handled, functionalized multilayers M(3), M(4), and M(5) were
prepared through the direct anchoring onto gold surfaces. For
comparison, terminal thiols 6, 7, and 8 were also prepared and
isolated in high yields by ligand exchange reaction from Pd(II)
chlorides in the presence of NaSH (see the Experimental

14734 J. Phys. Chem. A, Vol. 113, No. 52, 2009
Vitaliano et al.
Figure 4. ORTEP view of 5. Thermal ellipsoids are shown at 30%
probability level. Butylic tails of PBu3 and all hydrogen atoms are not
shown for clarity. i ) -x + 1, -y, -z + 1.
Figure 5. Crystal packing of 5, view along [100] direction.
Figure 2. Crystal packing of 2, view along [001] direction.
the metal through the S atoms. In the mononuclear complex 3,
a square-planar environment around the metal with two trans-
PBu3 and two thioacetate groups is present (angles are in the
range 86.4-93.4°). The two pendant oxygen atoms of the
thioacetate groups show the sterically more favorable anti
conformation, as observed for the analogous Pd(II) complex
trans-[Pd(kS-S{O}CMe)2(PPh3)2].46 The Pd-P [2.308(4) and
2.330(4) Å] and Pd-S [2.300(4) and 2.346(4) Å] bond lengths
fall in the range found for the above-mentioned compound and
are also consistent with the literature data.65
The molecular structure of 5 presents significant geometrical
similarities to 2. The molecule sits on a crystallographic center
of symmetry located at the midpoint of the C-C bond of the
biphenyl group, which is perfectly planar. The square planar
coordination geometry presents two trans PBu3 groups and the
thioacetate ligand with the acetylide moiety trans to each other.
As expected, the molecule is linear and shows an inclination of
83.5(3)° between the mean planes of the biphenyl moiety and
the metal coordination plane. All the bond distances and angles
are in agreement with those of complex 2. The only relevant
difference is a lengthening of the Pd-C1 bond distance trans
to the thioacetate group with respect to Pd-C1 trans to chloride
(2.01(1) Å to compare with 1.93(1) Å), probably attributable
to the major trans influence of the sulfur ligand with respect to
the chloride. The crystal packing of 5 presents two separate
domains, one containing the aliphatic moieties and the other
the aromatic ones. The molecules dispose in parallel layers, piled
along -b + c (Figure 5.).
UV-vis absorption and emission spectra of the organome-
tallic molecules have been recorded to investigate the structure/
properties relationship. The absorption maxima showed peaks
in the range 270-330 nm. Comparing the UV spectra of the
Pd(II) acetylyde complexes with those of the corresponding
organic precursors, that is, phenylacetylene (λmax ) 246 nm)
and DEBP (λmax ) 290 nm), a red shift of the UV absorption
was generally observed due to higher π delocalization. The UV
spectrum of complex 2 shows two absorption maxima at 275
and 328 nm; the latter one, the most intense, is due to the π f
π* transitions, and the first one, weaker, is due to atomic orbital
d-d transitions of the metal. The emission properties of Pd(II)
compounds were also investigated and showed emission maxima
in the range 300-450 nm with luminescence quantum yield
Figure 3. ORTEP view of 3. Thermal ellipsoids are shown at 30%
probability level. All the hydrogen atoms are not shown for clarity.
A partial conjugation can be observed, being C1tC2 and
C2-C3 bond lengths, respectively longer and shorter (1.213(15)
and 1.474(13) Å) compared to typical nonconjugated bonds.
The position of the halogen atom and of the acetylide moieties
is almost linear (Cl-Pd-C ) 177.7(4)°, Pd-CtC ) 177.3(1)°),
confirming the linearity of the whole molecule and, then, the
rigid rod nature of this model compound. The biphenyl moiety,
lying on the inversion center, is strictly planar, and its mean
plane is quite perpendicular to the coordination plane of the
metal [angle between their mean planes is 82.0(3)°]. The
complex 2 is essentially isostructural with the analogous Pt(II)
complex, recently described.63 No significant differences were
found in the coordination environment or in the geometric
parameters, in line with the similarity of the palladium and
platinum covalent radii.64 The crystal packing of 2 shows two
separate sets of molecules piled along c, tilted relative to each
other at about 70° (Figure 2).
The molecular structures of 3 and 5 obtained from X-ray
diffraction studies, are reported in Figures 3 and 4. It is worth
noting that in both complexes, the thioacetate groups coordinate

SAMs Based on Pd-Containing Organometallic Thiols
J. Phys. Chem. A, Vol. 113, No. 52, 2009 14735
values between 0.10 and 3.8%, the latter being worthy of notice
for organometallic molecules.
To better assess the complexes’ electronic structures, a low-
resolution X-ray photoelectron spectroscopy (XPS) investigation
was performed. C 1s, P 2p, Pd 3d, Cl 2p, and S 2p core level
spectra were collected, and the measured binding energy (BE)
values appeared consistent with those already published for Pd-
containing polymetallaynes35 and expected for the proposed
molecular structures (confirmed by XRD results). The core level
BE values (reported in the Supporting Information) are quite
close for samples bearing the same tail group, showing that the
configuration and electronic structure of the metal center is not
affected by the substitution of the terminal ligand (Cl, SH, or
SCOCH3), thus confirming that the overall molecular geometry,
that is, a square planar arrangement of the ligands around the
transition metal, is preserved in all samples.
In detail, the Cl 2p BE value for samples 1 and 2 was found
at 197.62 eV, which is the same as reported for similar Pd
complexes.66 The Pd 3d spectra collected for all the samples
here considered were also analyzed by curve fitting, and
appeared structured with two pairs of spin orbit components.
The main Pd 3d5/2 component found occurring at about 337.6
eV BE was attributed to Pd(II) tetracoordinated by comparison
with the Pd 3d5/2 signal of the pristine square planar Pd complex
(the Pd 3d5/2 peak in trans-[Pd(PBu3)2Cl2] has been recently
found at BE ) 338.1 eV). The observed decrease in the Pd 3d
BE of samples 1 and 2 with respect to the precursor complex
reflects an increase in charge density at the Pd sites and has
been ascribed to the presence of additional coordination of
acetylenic groups by the Pd atoms; in our case, belonging to
the DEBP moiety of a neighboring molecule in an intermolecular
assembling, as also suggested by literature data. The presence
of a negligible amount of Pd(0) was detected by the occurrence
of a minor 3d5/2 contribution at lower BE values (336.1 eV).
For samples 1 and 2, Cl/Pd intensity ratios equal to 1/1 were
estimated. The P 2p spectra of all samples show peaks at about
130.8 eV BE, in agreement with the values reported in the
literature for metal-bonded phosphine groups,67 and the Pd/P
atomic ratio is 1/2 for all the samples, as expected. For thiolate
and thiol ending group-containing compounds, the S 2p core
spectra appeared structured by two spin-orbit components.
The spectra have been analyzed by curve-fitting, obtaining the
expected BE separation of 1.20 eV as well as an atomic ratio
of 2/1 (S 2p3/2-S 2p1/2). The S 2p3/2 spin-orbit component for
thiolate samples 3, 4, and 5 was observed at 162.0 eV, a BE
value fully consistent with those reported in the literature for
sulfur-containing molecules interacting with metals through the
formation of a sulfur-metal chemical bond.68 The S/Pd atomic
ratios were estimated for all mononuclear and binuclear samples.
For complexes and thiol samples 6, 7, and 8, a value of about
1/1 was found, as expected from the proposed molecular
structure. For sample 5, a S/Pd value of about 2/1 was calculated,
which is in excellent agreement with the chemical structure
reported in Scheme 1. XPS results support the chemical structure
assessments obtained from the other characterizations and from
the XRD crystal structure analysis and confirm the stability of
the thiolate and thiol complexes toward X-ray exposition.
3.2. Multilayers and SAMs Achievement and Character-
ization. To establish the potentiality of these molecules as
candidates in electrical circuits, we have investigated their self-
assembly on gold-coated substrates and performed preliminary
experiments on the resulting multi- and monolayer films. The
low concentrations (typically 1-2 mM) of the solutions of
complexes that are used in the preparation of SAMs allowed
Figure 6. SAM preparation.
us to use different solvents. In our case, a THF solution of
thiolate precursors was used. Thiol substituents can directly form
self-assembled multilayers and monolayers on gold surfaces;69
however, because of the oxidative instability of organometallic
thiol groups, the thioacetyl protecting functionality was intro-
duced, and thiols were produced in situ by a deacylation
reaction. In situ monitoring by FTIR and UV techniques of the
deacylated thiol complex solutions was compared with the
solutions of on-purpose prepared terminal thiols 6, 7, and 8,
thus confirming the achievement of the terminal thiol function-
ality. The solution was allowed to react, then freshly cleaned
gold substrates were introduced. Self-assembled monolayers are
formed very rapidly on the substrate; however, it was necessary
to use adsorption times of 15 h or more to get multilayers M(3),
M(4), and M(5).
Neat SAM(3), SAM(4), and SAM(5) were obtained by
rinsing the substrates with different organic solvents. A sche-
matic outline of the SAM preparation procedure on gold
substrates is given in Figure 6.
To perform a preliminary evaluation of the coverage and
molecular orientation of the so-far-obtained multilayer samples,
IRRAS spectra were collected. The Au-S bond is strong and
contributes to the stability of the multilayers together with the
van der Waals forces between adjacent methylene groups. The
CH stretching vibrations of the alkyl chains are very sensitive
to packing density and to the presence of gauche defects, which
makes them ideally suited as probes to determine the quality
of the films.70 Our choice was to compare samples with a
different number of metallic centers (i.e. M(3) and M(5)) and
to study the behavior of the methylenic chains of PBu3 ligands.
In particular, the antisymmetric CH2 stretching vibration is a
useful indicator; comparing M(3) and M(5), a variation from
2926 to 2930 cm-1 was observed, together with a broadening
of the peak. A variation in the symmetric CH2 stretching
vibration was also observed, and M(3) and M(5) showed peaks
at 2960 and 2957 cm-1, respectively. These differences are
indicative of a lower-order degree of in the M(5), probably due
to a higher steric hindrance induced by the organic spacer and
from the presence of two metallic centers.
3.2.1. High-Resolution X-ray Photoelectron Spectroscopy.
As for the organometallic thiols discussed in this work, since
these materials are new, the acquiring of reliable information
on both molecular structures and properties of the SAMs formed
from these samples is a mandatory task for defining their self-
assembling behavior. HRXPS is very well suited to investigating

14736 J. Phys. Chem. A, Vol. 113, No. 52, 2009
Vitaliano et al.
Figure 7. (a, b, c) S 2p HRXPS spectra of monolayers SAM(3), SAM(4), and SAM(5); (d, e, f) S 2p HRXPS spectra of M(3), M(4), and M(5).
TABLE 1: HR-XPS Data for Multi- and Monolayers: M(3),
M(4), M(5), SAM(3), SAM(4) and SAM(5) on Gold
the molecule-substrate interaction in SAMs because the high
spectral resolution and photon flux allow one to establish the
chemistry occurring at the headgroup-substrate interface.
To ascertain that the SAM systems were efficiently obtained
by following the procedure described above and to investigate
the molecule-substrate interaction, high resolution XPS spectra
were collected for Au4f and S2p core levels. S 2p HRXPS
spectra of SAM(3), SAM(4), and SAM(5) are shown in Figure
7a, b, and c; the same spectra collected on M(3), M(4), and
M(5) are reported in Figure 7d, e, and f. The peak position,
fwhm, and relative intensities are reported in Table 1.
As expected, all the S 2p spectra show a spin-orbit doublet
(S 2p1/2, S 2p3/2); the more intense S 2p3/2 component is taken
as reference and reported in the table. The BE position of this
signal indicates whether the sulfur atom is covalently bonded
to the metal surface or it is not. For thiolate species chemisorbed
on metals, an S 2p3/2 BE value of nearly 162 eV is expected;67
S 2p3/2 signals around 163.5 eV are usually assigned to
physisorbed molecules at the SAM-ambient interface71 or
physisorbed thiols.72,29 For the SAM systems obtained from
sample 3, SAM(3), two S 2p spin-orbit doublets are observed
with the 2p3/2 component occurring at 162.0 eV and 163,7 eV.
BE
(eV)
fwhm atomic ratios est. thickness
sample/signal
SAM(3)
S 2p3/2 S-Au (S a)
S 2p3/2 SH (S b)
M(3)
(eV)
(S a/S b)
(nm)
0.2
162.0 1.58
163.7 1.58
1
1
S 2p3/2 SH (S b)
SAM(4)
163.2 1.83
1
0.4
1.1
S 2p3/2 S-Au (S a) 162.1 1.70
M(4)
S 2p3/2 SH (S b)
SAM(5)
163.0 1.90
S 2p3/2 S-Au (S a) 162.5 1.61
1
1
S 2p3/2 SH (S b)
M(5)
S 2p3/2 SH (S b)
163.6 1.61
163.2 1.61
The low-energy S 2p3/2 peak (S a) is ascribed to sulfur bonded
to gold, subsequent to the anchoring of the thiol on the metal
surface, whereas the high-energy S 2p3/2 signal (S b) is assigned
to sulfur in the free thiol terminal group. Since the relative
contributions of the different sulfur-containing species can be

SAMs Based on Pd-Containing Organometallic Thiols
J. Phys. Chem. A, Vol. 113, No. 52, 2009 14737
estimated from the ratio between the respective signal intensities
(peak areas), the calculated intensity ratio S a/S b ) 1/1 indicates
that 50% of the S-terminal groups in SAM(3) are covalently
bonded to the gold surface, and therefore, only one of the two
thiol-ending groups appears to be involved in the interaction.
The molecular structure (see Scheme 1) of SAM(3) suggests
that for steric effects due to the tributylphosphine groups array,
the molecules should graft the substrate surface with one sulfur
atom, whereas the second -SH group should remain free. The
above hypothesized molecular arrangement is fully supported
by the HRXPS S 2p spectra, reported in Figure 7a. In Figure
7d, the S 2p signal collected for sample 3 prior to rinsing (thick
layer M(3)) is also displayed. The multilayer spectrum shows
a pair of spin orbit components only, with an S 2p3/2 BE of
163.2 eV, a value consistent with physisorbed thiol molecules
and, therefore, two equivalent -SH groups, as expected for a
thick film.
Figure 8. NEXAFS C K-edge spectra of SAM(3), SAM(4), and
SAM(5) recorded in magic angle geometry.
Similar to SAM(3) and M(3), the S 2p spectrum for samples
SAM(5) (Figure 7c) and M(5) (Figure 7f) presents two pairs
of spin orbit components for the monolayer film and one pair
for the multilayer system. The S 2p3/2 BE values observed for
this sample are fully consistent with those already discussed
for SAM(3) and M(3), as shown in Table XPS. As for the
atomic ratios in the monolayer sample, an S a/ S b ratio value
of 1/1 was estimated. Assuming that the peak at 163.6 eV
originates from free thiol terminal groups, the observed intensity
ratio suggests that as for SAM(3), in the SAM(5) sample, the
molecules graft the gold surface with only one terminal group.
HRXPS spectra of monolayer and multilayer systems obtained
by grafting on gold sample 4 are also reported (Figure 7b and
e, respectively). For this sample, a single pair of spin-orbit
components is observed in both monolayer and multilayer
regimes. The S 2p3/2 signal for SAM(4) in the monolayer regime
occurring at about 162 eV indicates that all molecules are
covalently bonded to the gold surface. Conversely, the S 2p3/2
BE value of 164 eV observed for M(4) suggests, as already
discussed for samples M(3) and M(5), that the XPS signal arises
mainly from physisorbed molecules.
Figure 9. NEXAFS C K-edge spectra of complex trans-Pd(PBu3)2Cl2
thick film on gold and of M(3) grafted on the same substrate.
regimes, thus excluding the presence of multilayers and in
agreement with the expectations.
3.2.3. NEXAFS Spectra. NEXAFS spectra of SAM systems
obtained from samples 3, 4, and 5 were acquired at the C
K-edge. Measurements were performed by varying the incidence
angle of the linearly polarized (95%) synchrotron radiation on
the sample surface, from grazing (20°) to normal (90°), with
the aim to investigate the electronic structure as well as the
average orientation of molecules.
NEXAFS C K-edge spectra of SAMs 3, 4, and 5 recorded
at an X-ray incidence angle of 54.7° on the substrate (magic
incidence), for which the measured intensity distribution is
independent of the molecular orientation, are displayed in
Figure 8.
Through the XPS data analysis, the thickness of the sample
films can be estimated, exploiting the correlation between the
signal intensity and the coverage thickness. This feature, for an
overlayer adsorbed onto a substrate, can be calculated by
evaluating the attenuation of the substrate signal according to
the equation
I ) I0 exp(-d/λAu)
The peak assignment of the relevant spectral features can be
done by considerations correlated to the chemical structure and
comparison with literature data. The chemical structure of
molecules grafting the gold surface after depositing sample 3
is strictly similar to the inorganic Pd(II) coordination compound
trans-Pd(PBu3)2Cl2. In Figure 9, the C K-edge NEXAFS
spectrum of M(3) on gold is displayed; the spectrum measured
for the trans-Pd(PBu3)2Cl2 complex is also reported for com-
parison. Both spectra show a dominant feature at about 288 eV
and a less intense band at 288.90 eV, associated with C1s f
σ* C-H transitions due to the butyl groups, and a broadband
around 293 eV assigned to C1s f σ* C-C excitations of the
same butyl phosphyne moieties. The small feature occurring at
the low-energy side of the first resonance of the M(3) spectrum
is thought to be an artifact due to the monochromator; this is
not observed in thetrans-Pd(PBu3)2Cl2 complex that was
measured in a previous experiment at a different storage ring
(LURE-superACO-beamline SA72).
where I is the intensity of the substrate signal in the presence
of the overlayer, I0 the intensity of the same signal for the clean
surface, d is the overlayer thickness, and λAu is the inelastic
mean free path for gold. λAu is correlated to the kinetic energy
(KE) of the photoemitted electron by equation
λAu ) B(KE)1/2
where B ) 0.087 nm (eV)-1/2 for organic materials.73 Within
the limits of the method and the use of a B factor that is not
specifically appropriate, we find that the thickness values
calculated in this way for the three SAMs are 0.2, 0.4, and 1.1
nm for samples SAM(3), SAM(4), and SAM(5), respectively,
as reported in Table 1. By comparison with molecular dimen-
sions evaluated from the XRD data reported above, these values
are indicative for monolayer or submonolayer adsorption

14738 J. Phys. Chem. A, Vol. 113, No. 52, 2009
Vitaliano et al.
Figure 11. Scheme reporting the angles used to determine the
geometrical arrangement of SAM(4) and SAM(5) on the substrate.
As shown in Figure 10, the C K-edge spectra were acquired
at grazing and normal geometry; that is, the incidence angle of
the linearly polarized light was varied from 20° to 90°. The
NEXAFS resonances intensity depends on the orientation of
the transition dipole moment (µ) of the probed molecules relative
to the polarization vector (p) of the incoming radiation. If the
directions of the electric field of the incoming radiation (E) and
µ are parallel to each other, the intensity is at the maximum.
On the other hand, the excitation does not occur when the
vectors E and µ are orthogonal to each other.75 The intensity
of the first and second features observed in the SAM 4 and 5
spectra (see Figure 10) shows a small but clear variation with
the beam incidence angle (linear dichroism), suggesting a high
level of molecular organization. Sample SAM(3) does not show
angular dependence effects, as predictable on the basis of its
molecular structure.
The average tilt angle of SAMs 4 and 5 (i.e., the angle
between the molecular axis and the normal to the substrate)
was obtained by the quantitative analysis of the angular
dependence of the first π* resonance (attributed to C 1s-π*
aromatic ring transition) intensity, following the procedure
reported by Sto¨hr.75 For the SAM(4) sample (one EP unit), the
average tilt angle was calculated by applying the simple formula
Figure 10. Polarization-dependent NEXAFS C K-edge spectra of
SAM(5), SAM(6), and SAM(7). The spectra have been recorded at
grazing (20°) (O) and normal (90°) (b) photon incidence angles; the
difference spectra (grazing - normal) are also displayed (black lines).
I(θ, R) ) 1 + (1/2)(3 cos2 θ - 1)(3 cos2 R - 1)
where angle R is the angle between the π* vector orbital and
the normal to the surface plane (in this case, the average tilt
angle of molecular axis, ꢀ, is simply 90° - R), and θ is the
incidence angle of the X beam on the sample. A schematic
drawing reporting the angles used to determine the molecular
arrangement on the substrate is reported in Figure 11.
Molecules of SAM(5) contain the DEBP unit in which the
two aromatic rings are twisted at an angle of R ) 42°72 with
respect to the plane defined by the surface normal and the
molecular axis. In this case, a more appropriate formula was
used. The term “cos R” can be expressed through the angle ν
and the average tilt angle ꢀ of the molecular axis with respect
to the normal to the substrate:
Following a similar approach, the assignment of peaks for
the C K-edge spectra of mono and multilayers on gold, obtained
from samples 4 and 5, that are alike due to the analogous
chemical groups (Pd-ethynnyl benzene-SH, Pd2-diethynyl
biphenyl-(SH)2), can be made by analogy with Pd-DEBP2.74
The main feature occurring at 285.50 eV is assigned to the
superimposition of the nearly unaltered C1s f π* transition of
the benzene ring, due to the ring carbons’ not being bonded to
the alkyne group and the low energy contribution of the
acetylene carbons. The signal at about 286 eV originates from
the higher-energy excitation of the alkyne carbons (CtC) and
includes a contribution from the ring carbon bonded to the
acetylene functional group. The resonance at nearly 288 eV is
related to σ*C-H due to the butyl groups, and the feature at
about 288.90 eV is thought to be derived by the overlapping of
a high energy π* plus σ*C-H. The broad bands around 293 is
assigned to σ*C-C, and the one at 303 eV is CdC benzene-
like. The expected feature above 310 eV, and originating from
the σ*CtC, did not show up from the spectral background.
cos R ) cos V sin ꢀ
I(θ, R, V) ∼ 1 + (1/2)(3 cos2 θ - 1)(3 cos2 θ sin ꢀ - 1)
(See ref 29 regarding the latter of the above equations.)
Following this procedure, tilt angles, ꢀ, of 40° and 30° were

SAMs Based on Pd-Containing Organometallic Thiols
J. Phys. Chem. A, Vol. 113, No. 52, 2009 14739
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obtaining a thickness of 1.8 nm for SAM(5) and 1.0 for
SAM(4). Since the SAM(3) molecules results are not
oriented, the same calculation for this system was not
performed. These values are overestimated due to the
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4. Conclusions
Multilayers and self-assembled monolayers of organometallic
thiols in situ prepared starting from organometallic thiolates,
that is, trans-[Pd(PBu3)2(SCOCH3)2], trans-[(C6H5CtC)Pd-
(PBu3)2(SCOCH3)], and trans,trans-[(CH3COS)Pd(PBu3)2-
(CtC-C6H4-C6H4-CtC)(PBu3)2Pd(SCOCH3)], were depos-
ited onto gold surfaces by a dipping procedure. The self-
assembly of the thiols on the gold surface was assessed, and
the HR-XPS measurements joined with NEXAFS studies
allowed us to evaluate the anchoring of the organometallic
moieties through the sulfur linkage to gold. Depending on the
investigated molecule, the interaction occurring at the interface
and the molecular orientation of the thiols on the surface with
tilt angles (defined as the angle between the molecular axis and
the normal to the surface) of about 30-40° was observed. The
thickness of the obtained SAMs was calculated to be 0.2, 0.4,
and 1.1 nm for samples SAM(3), SAM(4), and SAM(5),
respectively. By comparison with molecular dimensions evalu-
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monolayer or submonolayer adsorption regimes. NEXAFS
studies suggested a high level of molecular organization for
SAMs 4 and 5. These SAMs offer a promising perspective for
the applications of these materials in nanoelectronics.
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Acknowledgment. The authors gratefully acknowledge the
financial support of University La Sapienza “Ateneo 2007”.
Supporting Information Available: Chemical synthesis
details and XPS characterization data of mono and binuclear
Pd(II) complexes, together with crystallographic data table and
CIF files for complexes 2, 3, and 5 are supplied. This material
is available free of charge via the Internet at http://pubs.acs.org.
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