H2S gas interaction with Pt(II)-containing polymetallaynes of selected chain length: An XPS and EXAFS study

Article abstract of DOI:10.1021/jp803119j

The interaction between gaseous H₂S and the surface of several metal-containing oligomers, investigated by emission and absorption spectroscopies, is presented and discussed. The polymetallayne trans- {Cl[Pt(PBu₃)₂(C=C-C₆H₄-C ₆H₄-C=C)]₉Pt(PBu₃)₂Cl} and related model molecules, i.e. the binuclear transition metal dialkynyl bridged Pt(II) square planar complex trans,tranA-[ClPt(PBu₃) ₂(C=C-C₆H₄-CoH₄-C=C)-Pt(PBu ₃)₂Cl], the tetranuclear linear oligomer trans-{Cl-[Pt(PBu₃)₂(C=C-C₆H₄-C ₆H₄-C=C)]₃Pt(PBu₃)₂Cl}, the tetranuclear cyclic oligomer cis-[Pt(PBu₃)₂(C=C-C ₆H₄-CoH₄-C=C)J₄, were exposed to hydrogen sulfide and then investigated by X-ray photoelectron (XPS) and X-ray absorption (XAS) spectroscopies, in order to shed light on the gas/polymer interaction associated to the sensing properties of these materials. XPS measurements evidenced the presence of S in the polymetallayne samples exposed to H₂S, and the measured S2p binding energy values correlate with H₂S adsorbed by means of sulfur atoms chemically bonded to metal atoms, owing to the formation of sulfur-containing adducts. XAS data analysis suggested a square-pyramidal geometry around the transition metal with H ₂S in the apical position for the pentacoordinated platinum units.

Full text of DOI:10.1021/jp803119j

J. Phys. Chem. A 2008, 112, 7365–7373
7365
H2S Gas Interaction with Pt(II)-Containing Polymetallaynes of Selected Chain Length:
An XPS and EXAFS Study
Chiara Battocchio,*,† Ilaria Fratoddi,‡ Maria Vittoria Russo,‡ and Giovanni Polzonetti†
Department of Physics and Unita` INSTM and CISDiC, UniVersity “Roma Tre”, Via della Vasca NaVale,
84 - 00146 Rome, Italy, and Department of Chemistry, UniVersity of Rome “La Sapienza”,
P.le A. Moro 5 - 00185 Rome, Italy
ReceiVed: April 10, 2008; ReVised Manuscript ReceiVed: June 3, 2008
The interaction between gaseous H2S and the surface of several metal-containing oligomers, investigated by
emission and absorption spectroscopies, is presented and discussed. The polymetallayne trans-{Cl-
[Pt(PBu3)2(Ct C-C6H4-C6H4-Ct C)]9Pt(PBu3)2Cl} and related model molecules, i.e. the binuclear transition
metal dialkynyl bridged Pt(II) square planar complex trans,trans-[ClPt(PBu3)2(Ct C-C6H4-C6H4-Ct C)-
Pt(PBu3)2Cl], the tetranuclear linear oligomer trans-{Cl-[Pt(PBu3)2(Ct C-C6H4-C6H4-Ct C)]3Pt(PBu3)2Cl},
the tetranuclear cyclic oligomer cis-[Pt(PBu3)2(Ct C-C6H4-C6H4-Ct C)]4, were exposed to hydrogen sulfide
and then investigated by X-ray photoelectron (XPS) and X-ray absorption (XAS) spectroscopies, in order to
shed light on the gas/polymer interaction associated to the sensing properties of these materials. XPS
measurements evidenced the presence of S in the polymetallayne samples exposed to H2S, and the measured
S2p binding energy values correlate with H2S adsorbed by means of sulfur atoms chemically bonded to
metal atoms, owing to the formation of sulfur-containing adducts. XAS data analysis suggested a square-
pyramidal geometry around the transition metal with H2S in the apical position for the pentacoordinated
platinum units.
Introduction
Organometallic Pt-containing macromolecules such as square-
planar Pt(II) complexes tend to coordinate sulfur-containing
molecules, giving rise to pentacoordinated adducts; actually,
sulfur dioxide is a versatile ligand in organic and inorganic
chemistry, as it can be active as both a Lewis base or acid. For
example, a series of square-planar Pt(II) complexes has been
successfully tested as SO2 sensors and found highly selective
and particularly selective for submillimolar to molar gas
quantities.7 Solid-state crystal structure of the SO2 adducts
showed a square-pyramidal geometry around the metal center
with SO2 in the apical position.7
Because of its electronic structure, hydrogen sulfide is
expected to interact with Pt centers in square-planar Pt(II)
complexes as an electron donor (Lewis base); in addition, a
fine-tuning of the electrophilicity of the metal center, by
electronic and steric modifications of the ligand array in the
pristine complex, would predictably influence the H2S coordina-
tion, thus influencing the reversibility of the linkage and the
optical response of the new molecular adduct.
Since the past decade, the electronic, optical, and liquid crystal
applications of “rigid-rod” organometallic polymers, obtained
from transition-metal complexes with alkynyl ligands, have been
thoroughly investigated,8 and the correlated intrachain electron
and hole migration of model Pt alkynyl mixed valence com-
plexes have been elucidated,9 as well as the charge transport in
molecular model Pt acetylides,10 in view of their applications
as components for molecular electronics. The research of our
group has been focused on investigations of the chemical and
electronic structure of Pt-containing rod-like organometallic
polymers11 and also diethynyl organic spacers that are the
building blocks of these materials.12 In this framework, binuclear
complexes and small oligomers have been successfully used as
model molecules for the interpretation of the optoelectronic
properties of more complicated systems.13 In the field of
Sulfur-containing compounds are common impurities in
fossil-derived fuels and chemical feed stocks1 and damage the
quality of the air by forming sulfur oxides SOx during the
burning of fuels, which are major air pollutants leading to acid
rain.2 In addition, the sulfur impurities are responsible for the
rapid deactivation or poisoning of most catalysts and for the
corrosion of equipment used in the chemical and petrochemical
industries.3 Therefore, the detection and monitoring of sulfur-
containing compounds are highly attractive.4,5 Among others,
hydrogen sulfide (H2S) is a toxic and malodorous gas, contained
in oil or natural gas mines, that has been proved very harmful
to the human body and to the environment and whose detection
and monitoring are of high importance for both resource
exploitation and human health. In recent times, a number of
semiconductor sensors have been found to be sensitive to H2S,
including SnO2, WO3, In2O3, ZnO2, a few perovskite-type
materials such as NdFeO3 or NiFeO4, and Pt-doped R-Fe2O3
powders.6 The application of these sensors is however limited
by some disadvantages such as the poor selectivity, long
response time, and high operating temperature or limited
detection range.
Traditionally used semiconducting materials are being re-
placed with organic and organometallic compounds in specific
application as the one presently discussed. The low cost,
processability, and versatility of organic and organometallic
oligomers and polymers are, among others, the parameters that
make these materials convenient and advantageous for replacing
the traditional ones.
* Corresponding author. Telephone: (+) 39-06-55173388, fax: (+) 39-
06-55173390, e-mail: battocchio@fis.uniroma3.it.
† University “Roma Tre”.
‡ University of Rome “La Sapienza”.
10.1021/jp803119j CCC: $40.75
2008 American Chemical Society
Published on Web 07/23/2008

7366 J. Phys. Chem. A, Vol. 112, No. 32, 2008
Battocchio et al.
technological applications, sensor devices based on sensitive
membranes made from rigid rod organometallic molecules in
the form of self-assembled (SAMs) monolayers can be suc-
cessfully implemented. For example, Pt polyynes have been used
as thin film membranes in surface acoustic wave (SAW)
devices14 showing high sensitivity toward relative humidity and
sulfur-containing organic vapors.15 Recent studies on sensors
based on analogue polymetallaynes showed a higher sensitivity
toward low relative humidity percentages, when nanostructured
membranes were employed.16 The obtained materials have been
extensively studied and conveniently used as sensors; however,
the basic understanding of some chemical and physical aspects
must still be investigated.
In this paper, we report information achieved about the
interaction occurring between H2S and a series of Pt-containing
molecules. The here considered samples are the binuclear
transition metal dialkynyl bridged Pt(II) complex trans,trans-
[ClPt(PBu3)2(Ct C-C6H4-C6H4-Ct C)Pt(PBu3)2Cl] (1), the
tetranuclear linear oligomer trans-{Cl-[Pt(PBu3)2(Ct C-C6H4-
C6H4-Ct C)]3Pt(PBu3)2Cl} (2), the tetranuclear cyclic oligomer
cis-[Pt(PBu3)2(Ct C-C6H4-C6H4-Ct C)]4 (3), and the multi-
nuclear(10Pt-containingunits)linearoligomertrans-{Cl-[Pt(PBu3)2-
(Ct C-C6H4-C6H4-Ct C)]9Pt(PBu3)2Cl} (4). These systems
were investigated by a combined X-ray photelectron (XPS) and
X-ray absorption (XAS) spectroscopy probing the electronic
structure of both oligomer and adsorbate molecules as well as
the geometric arrangement of the whole system.
On the basis of previously achieved results concerning the
specific sensing properties of platinum poly ynes to H2S,15 in
the framework of an extensive investigation as a starting
approach we have considered reaction conditions that guarantee
the grafting of H2S to the Pt-containing systems; however, our
planning considers further investigations in more controlled
exposure conditions.
Figure 1. Molecular structures of trans,trans-[ClPt(PBu3)2-
(C≡C-C6H4-C6H4-C≡C-Pt(PBu3)2Cl] (1), trans-{Cl-[Pt(PBu3)2-
C≡C-C6H4-C6H4-C≡C]3 Pt(PBu3)2 Cl} 2, cis-{[Pt(PBu3)2-
C≡C-C6H4-C6H4-C≡C]3 Pt(PBu3)2] (3), and trans-{Cl-
[Pt(PBu3)2C≡C-C6H4-C6H4-C≡C]9 Pt(PBu3)2 Cl} (4).
Experimental Section
Materials and Methods. Complex 1 and oligomers 2 and 4
were synthesized as reported in previous papers,17,18 by fol-
lowing a dehydrohalogenation procedure, involving the reaction
of trans-[Pt(PBu3)2Cl2] square-planar complexes with the 4,4′-
diethynylbiphenyl (DEBP) monomer in the presence of diethy-
lamine as the solvent. Complex 3, cis-{[Pt(PBu3)2Ct C-C6H4-
C6H4-Ct C]4, was prepared by slightly changing the dehydro-
halogenation reaction, and the experimental conditions and main
spectroscopic characterizations are reported:
equipped with input and output gas lines, that allowed EXAFS
measurements to be performed on samples in controlled
chemical environment. Samples were finely ground and mixed
with cellulose; the powder was then pressed by means of a
hydraulic press equipped with a Specap P/N 10 mm evacuable
pellet die. The pellets of samples 1-4 were successively exposed
to H2S gas, and a characteristic color change, for all of the
samples, from white/light yellow to orange/brown was indicative
for the gas absorption producing a chemical reaction then leading
to the formation of a new molecular species.
A 0.40 g (0.6 mmol) amount of cis-[Pt(PBu3)2Cl2] and 0.12 g
(0.6 mmol) of DEBP were dissolved in 30 mL of diethylamine,
CuI was then added (5.0 mg, 0.03 mmol), and the reaction was
run at T ) 25 °C for 3 h. From the reaction mixture, the product
3 was extracted from CH2Cl2/H2O, and the organic phase was
dried and then purified by chromatography on SiO2 with an
eluant mixture petroleum ether/THF 5/1. The eluted fraction
containing compound 3 was dried and characterized (0.39 g,
Spectroscopic Characterizations. FTIR spectra were re-
corded as nujol mulls or as films deposited from CHCl3 solutions
by using CsI cells, on a Bruker 70 Fourier transform spectrom-
1
eter. H and 31P NMR spectra were recorded in CDCl3 on a
Bruker AC 300P spectrometer at 300 and 121 MHz, respec-
tively; the chemical shifts (ppm) were referenced to TMS for
1H NMR assigning the residual 1H impurity signal in the solvent
at 7.24 ppm (CDCl3). 31P NMR chemical shifts are relative to
H3PO4 (85%). Elemental analyses were performed at the
Department of Chemistry, University of Rome ”La Sapienza.
UV-vis spectra were recorded on a Cary 100 Varian
instrument. All measurements were performed at room tem-
perature using quantitative CHCl3 solutions of the samples
before and after H2S exposition.
0.12 mmol, yield 63%): UV (nm, CHCl3): 340.8; FTIR (cm-1
,
nujol mulls): 2113 (ν Ct C), 1901, 1602 (ν CdC); 1H NMR (δ
ppm, CDCl3): 0.91 (t, CH3), 1.42 (q, CH2CH2CH3), 1.51 (m,
CH2CH2CH3), 2.01 (m, PCH2), 7.41 (ArH),7.28 (ArH); 31P
NMR (δ ppm, CDCl3): -2.43, (JP-Pt ) 2249 Hz); Elemental
analyses, found (calculated) for C160H248P8Pt4 (%): C ) 59.42
(60.22), H ) 8.01 (7.78). Mass spectrum: 3191 m/z. (calculated
3188).
Adduct Formation with Hydrogen Sulfide. The exposure
of complex 1 and oligomers 2, 3, and 4 to 500 mbar of H2S
(Air Liquide, 99.95%) was carried out in a chemical cell
X-ray Photoelectron Spectroscopy. XPS analysis was
performed in an instrument of our own design and construction,
consisting of a preparation and an analysis chamber, equipped

H2S Interaction with Pt(II)-Containing Polymetallaynes
J. Phys. Chem. A, Vol. 112, No. 32, 2008 7367
samples only. XPS data collected on pellets and films (i.e., with
and without cellulose) gave identical S2p, P2p, Pt4f, Cl2p core
level spectra.
Extended X-ray Absorption Fine Structure (EXAFS).
EXAFS21 experiments were performed at ESRF storage ring at
the GILDA CRG beamline. The monochromator was equipped
with two Si(311) crystals,22 and harmonic rejection was achieved
by using a pair of Pd-coated mirrors with a cutoff energy of
20.2 keV. Data were collected in transmission mode. For the
X-ray absorption spectroscopy (XAS) measurements, pellet
samples of the macromolecules mixed with cellulose were
prepared.
EXAFS measurements were initially performed in low
vacuum conditions (P ) 10-3 mbar) on the pristine samples.
Removal of contaminants in the chemical cell system was
achieved by several cycles of pumping and filling with H2S; a
final filling with hydrogen sulfide was then made up to a partial
pressure of about 500 mbar, and EXAFS structural characteriza-
tion was carried out again. In situ treatments could be performed
at the GILDA beamline by means of a small chemical cell
equipped with a gas line, built on purpose.23 In the framework
of our study, as a first step the choice of high gas pressure in
the chemical cell (500 mbar) was made in order to guarantee
that a chemical interaction arises between the organometallic
molecule and H2S gas, leading to the formation of an adduct
whose molecular structure study is the main topic of this
research work at this stage. EXAFS measurements in H2S
environment started as soon as the cell was filled up with gas,
and at least four EXAFS spectra were collected on each sample.
A good resolution EXAFS spectrum at the Pt-LIII-edge require
nearly 45 min to be collected; therefore, we can estimate that
each sample was exposed to H2S for about 2 h. However, a
comparison between spectra recorded in sequence on the same
sample showed that these were identical to each other; this result
was indicative for an interaction between Pt-containing materials
and hydrogen sulfide relatively fast that gives rise to a chemical
species quite stable in the reported experimental conditions.
Figure 2. Pt4f XPS spectra collected on sample 4 prior and after
exposure to H2S gas. For all the treated samples, the overall signal is
wider, and a small asymmetry, arising by the new spectral component,
is detectable at lower BE. It is noteworthy that the component ascribed
to the pristine Pt poly yne (Pt4f7/2b, higher BE values) in the spectrum
collected on the treated sample shows the same BE and fwhm values
of the pristine sample spectrum. BE and fwhm values for Pt4f signals
of all samples, both pristine and interacting with H2S (b component),
are reported in the included table.
with a 150 mm mean radius hemispherical electron analyzer
with a four-elements lens system with a 16-channel detector
giving a total instrumental resolution of 1.0 eV as measured at
the Ag 3d5/2 core level. Mg KR nonmonochromatized X-ray
radiation (hν ) 1253.6 eV) was used for acquiring core level
spectra of both substrate and adsorbate samples (C1s, P2p, Pt4f,
Cl2p, and S2p). The spectra were energy referenced to the C1s
signal of aromatic C atoms having a binding energy BE )
285.00 eV. Atomic ratios were calculated from peak intensities
by using Scofield’s cross-section values and calculated λ
factors.19 Curve-fitting analysis of the C1s, P2p, Pt4f, S2p, and
Cl2p spectra was performed using Voigt profiles as fitting
functions, after subtraction of a Shirley-type background.19,20
XPS measurements were carried out on complex 1 and oligo-
mers 2, 3, and 4 before and after gas exposure. Measurements
on the pristine samples were performed on thin films, obtained
by spin-depositing onto Si(111) wafers surfaces at 700 rpm for
10 s dichloromethane solutions of 1-4. H2S exposure was
performed, as described in section 2.1, on solid samples mixed
with cellulose and a first series of XPS data was collected on
these pellets. As a purpose of comparison, X-ray photoelectron
spectroscopy was also performed on thin films obtained by
spinning dichloromethane solutions of samples 1-4, after H2S
exposure, on Au/Si(111) substrates. CH2Cl2 solutions of samples
1, 2, 3, and 4 after H2S exposure were obtained by percolating
the solvent through the cellulose-containing pellets. Since the
cellulose is not dissolved by dichloromethane, the solutions
obtained following this procedure contain the organometallic
XAS data in the so-called extended region were extracted
with the AUTOBK code24 by linear fitting of the pre-edge region
and subtraction of the atomic background fitted with a cubic
spline. Theoretical XAS signals were generated using ARTE-
MIS25 software with FEFF8.2026 calculations, starting from the
cluster obtained by X-ray diffraction measurements of the trans-
[ClPt(PBu3)2(Ct C-C6H4-C6H4-C≡C-Pt(PBu3)2Cl] sample.
Results and Discussion
Pristine Materials Characterization. The dinuclear complex
trans,trans-[ClPt(PBu3)2(Ct C-C6H4-C6H4-Ct C)Pt(PBu3)2-
Cl] (1), the tetranuclear linear oligomer trans-{Cl-
[Pt(PBu3)2(Ct C-C6H4-C6H4-Ct C)]3Pt(PBu3)2Cl} (2), the
tetranuclear cyclic oligomer cis-[Pt(PBu3)2(Ct C-C6H4-
C6H4-Ct C)]4 (3), and the multinuclear (10 Pt units) linear
oligomer trans-{Cl-[Pt(PBu3)2(Ct C-C6H4-C6H4-Ct C)]9-
Pt(PBu3)2Cl} (4), whose molecular structures are reported in
Figure 1, were fully characterized by means of XPS, EXAFS,
and UV-visible absorption spectroscopies before exposure to
H2S. Among them, the novel compound 3 was isolated and fully
characterized. The formation of a cyclic structure was achieved
in analogy with analogous Pt(II) complexes27 and confirmed
by means of mass spectrometry and NMR spectroscopy. In
particular, in the 31P NMR spectrum, a signal at -2.43 ppm
was found, with a coupling constant JP-Pt equal to 2249 Hz,
typical of cis square-planar complexes.28

7368 J. Phys. Chem. A, Vol. 112, No. 32, 2008
Battocchio et al.
TABLE 1: EXAFS Data Analysis Results for Pristine Samples 1-4a
scattering path
1
2
3
4
single scattering Pt-C1
R
1.96(2)
45(5)
2.31(3)
30(4)
1.99(5)
81 (6)
2.29(7)
81(6)
1.96(2)
45(5)
2.31(3)
30(4)
2.00(8)
29(4)
2.31(2)
27(2)
σ × 104
single scattering Pt-P
R1
σ1 × 104
R2
single scattering Pt-C2,
double scattering Pt-C2-C1,
triple scattering Pt-C1-C2-C1
3.16(8)
3.19(4)
3.16(8)
3.21(4)
σ2 × 104
R3
72(2)
113(1)
1.97(9)
84(6)
72(2)
48(4)
-
-
single scattering Pt-Cl
2.26(4)
826(5)
0.010
2.26(4)
826(5)
0.020
σ3 × 104
R-factor
0.018
0.020
a R is in angstroms, σ is in 10-4 Å2. The numbers in parentheses are the uncertainties in the interatomic distances and DW factors. EXAFS
data for 1 and 4 (ref 18) are also reported for comparison. For each sample, the global amplitude factor S02 and the edge position ∆E were
0
fixed to the best-fit values and are the same for all the considered paths. R-factor is a least-squared residual, showing the goodness of fit.
X-ray photoelectron spectroscopy and UV-visible absorption
measurements of samples 1, 2, and 4 have been already
performed and discussed by some of us,17 giving evidence of
the electronic delocalization through the transition metal 6p and
5d orbitals, that was expected on the basis of theoretical studies
on the band structure of organometallic polymetallaynes.29
Recently, XPS measurements of C1s, Pt4f, and P2p core
levels have been also performed on the newly synthesized
sample 3, and the observed spectra are completely similar to
those acquired on samples 1, 2, and 4 and discussed in reference
17 as expected. In fact, the Pt4f7/2 spin orbit component peak
of sample 3 occurs at 73.10 eV binding energy (BE), with a
full width at half maximum (fwhm) of 2.03 eV and is therefore
fully consistent with Pt4f7/2 components already observed for
1, 2, and 4 (see Figure 2 and included table). The evaluation of
atomic ratios by means of XPS confirmed the calculated
stoichiometry and allowed to assess the number of repetitive
units of all samples; furthermore, measurements performed after
aging the samples for several months lead to establish the
environmental stability of these Pt-containing materials.17
In addition, a joint X-ray diffraction and EXAFS study was
carried out on complex 1,18 leading to assessment of the square-
planar geometry of the metallic center. The XRD diffraction
parameters obtained for sample 1 were also used at that time
as a basis set for the elaboration of EXAFS data collected on
the ten units oligomer 4, i.e. trans-{Cl-[Pt(PBu3)2-
Ct C-C6H4-C6H4-Ct C]9Pt(PBu3)2Cl}, whose crystal struc-
ture could not be obtained because of its amorphous nature,
and allowed us to establish the square-planar geometrical
arrangement of the acetylene moieties and of the tributyl
phosphine groups around the transition metal. Referring to this
model, the Pt atom is four-coordinated with two P atoms (which
in the full molecule are tributylphosphine units) and two
acetylene units (that belong to the diethynyl-biphenyl moieties
DEBP).
In the present work, the method of data analysis previously
developed18 has been fruitfully applied in order to investigate
the molecular structure of samples 2 and 3. In order to analyze
the EXAFS data collected on samples 2 and 3 at the Pt-LIII-
edge, i.e. in the 11400-12600 eV energy region, contribution
to the total signal from the P1, P2, C1, C2, C11, and C21 atoms
have been considered by including the single (Pt-C1, Pt-C2,
Pt-C3, Pt-P1), double (Pt-C1-C2) and triple (Pt-C1-C2-C1)
linear scattering paths for the photoelectron. In order to describe
the single scattering (SS) involving the Pt, C1, P, C2, and Cl
atoms, i.e. Pt-C1, Pt-P, and Pt-C2, respectively, the bond
length and Debye-Waller (DW) factors R, σ, R1, σ1, R2, σ2,
R3, σ3 were used; each path has a 2-fold degeneration, reflecting
the presence of two P and two acetylene groups around Pt. The
multiple scattering (MS) paths Pt-C2-C1-Pt and Pt-C1-
C2-C1-Pt were also considered with parameters R2, σ2. This
is justified by the fact that the atomic configuration Pt-C1-C2
is collinear so the path of the SS and MS paths are strictly the
same. The use of a common DW factor is an approximation
that permits a limited number of free parameters. Fits of the
data have been carried out in R space after Fourier transform
of the k weighted EXAFS data in the range k ) 3.0-11.5 Å-1
,
while the R range for the fits was R)1.1-4.0 Å. S02 and ∆E0
have been fixed to the best-fit values, and the resulting values
for the atomic distances R, R2, R3 and relative Debye-Waller
factors σ, σ2, σ3 are listed in Table 1. The overall quality and
suitability of the fits was also evaluated by ARTEMIS25 software
and is shown as the “R-factors” in Table 1. In the present case
R factors in the range 0.010-0.020 have been obtained, thus
evidencing the goodness of the analysis; as a rule it is generally
Figure 3. (a) EXAFS signal weighted with k3 for 2 and 3 and respective
best fits; (b) Fourier transformation of the EXAFS signal in R space
and best fits for 2 and 3.

H2S Interaction with Pt(II)-Containing Polymetallaynes
J. Phys. Chem. A, Vol. 112, No. 32, 2008 7369
TABLE 2: XPS Data Collected on Samples 1-4 after Exposure to H2S Gas
pelleta
BE (eV)
CHCl3 solution
BE (eV)
fwhm (eV)
A/σ
fwhm (eV)
A/σ
average atomic ratios
Complex1
C1s
P2p3/2
285.00
131.23
72.43
73.22
162.26
198.45
285.00
131.23
72.34
73.10
162.29
198.47
1.87
1.86
1.94
1.94
2.07
1.68
4713
301.9
44.0
79.5
42.8
C/Pt(tot) ) 38.2
P/Pt(tot) ) 2.2
Pt(a) ) 35%
1.88
1.96
1.96
2.14
1.74
264.5
42.8
78.9
45.1
97.9
Pt4f7/2
Pt(b) ) 65%
S2p3/2
Cl2p3/2
Pt(a)/S ≈ 1.0
101.4
Cl/Pt(tot) ) 0.8
Oligomer 2
C1s
P2p3/2
285.00
131.23
72.28
73.09
162.23
198.13
285.00
131.23
72.33
73.13
162.03
198.56
1.91
2.03
1.89
1.89
2.06
1.82
5037
251
34.0
67.9
35.4
64.0
C/Pt(tot) ) 51.4
P/Pt(tot) ) 2.4
Pt(a) ) 33%
1.90
1.92
1.92
1.79
2.02
169
Pt4f7/2
32.4
59.3
29.9
47.0
Pt(b) ) 67%
S2p3/2
Cl2p3/2
Pt(a)/S ) 1.0
Cl/Pt(tot) ) 0.5
Oligomer 3
C1s
P2p3/2
285.00
131.21
72.45
73.25
162.15
285.00
131.21
72.38
73.09
162.32
1.70
1.99
1.92
1.92
2.48
3917
164
25.1
52.5
27.8
C/Pt(tot) ) 50.5
P/Pt(tot) ) 2.2
Pt(a) ) 34%
Pt(b) ) 66%
Pt(a)/S ≈ 1.0
2.06
1.95
1.95
2.14
120
Pt4f7/2
17.9
32.8
18.0
S2p3/2
Oligomer 4
C1s
P2p3/2
285.00
131.28
72.41
73.26
162.38
285.00
131.28
72.42
73.13
162.29
1.87
1.98
1.93
1.93
2.25
3322
163.2
27.5
52.7
31.7
C/Pt(tot) ) 41.4
P/Pt(tot) ) 2.0
Pt(a) ) 34%
Pt(b) ) 66%
Pt(a)/S ) 1.0
1.88
1.92
1.92
2.63
155
Pt4f7/2
30.8
60.0
27.0
S2p3/2
a The main component of XPS C1s spectra collected on pellets is the cellulose signal.
accepted that the fit is reliable for R factor values lower than
0.050.30 EXAFS spectra in K space (ꢀ(k) × K3) and the related
Fourier transforms (ꢀ(R)) are reported together with the relative
best fits in Figure 3a and Figure 3b, respectively
responsible for the well-known sensitivity of Pt poly ynes toward
sulfur-containing molecules.14–16 Pt4f XPS spectra as collected
on 1-4 pellet samples after exposure to H2S, are displayed in
Figure 4.
Best fit results that are collected in Table 1 show a very slight
difference between atomic distances and DW factors already
found for complex 1 and oligomer 4 and the same parameters
estimated for samples 2 and 3, confirming that the square-planar
geometrical arrangement of the acetylene moieties and of the
tributyl phosphine groups around the transition metal already
assessed for the model molecule 1 and oligomer 418 are
preserved in both tetranuclear samples.
Samples Characterization during and after Interaction
with H2S. Complex 1 and oligomers 2, 3, and 4 were exposed
to H2S gas as reported in the Experimental Section. EXAFS,
XPS, and UV-visible absorption measurements have been
performed on all the samples after gas treatment, and the
comparison between the obtained spectra and those measured
for the pristine materials lead to ascertain that are the Pt centers
undergoing chemical perturbation being involved in the interac-
tion of Pt poly ynes with H2S.
XPS. XPS measurements of C1s, P2p, Cl2p, S2p, and Pt4f
core levels were carried out after H2S exposure on both cellulose
pellets and thin films (obtained by spinning CH2Cl2 solutions
on Au/Si(111) substrates) for all the four samples considered
here; the two series of experiments (cellulose pellet and thin
film) gave the same results, indicating that cellulose does not
play any role during the polymer interaction with H2S. XPS
data (BE, fwhm, atomic ratios) collected on complex 1 and
oligomers 2, 3, and 4 either as pellets or as solutions are reported
in Table 2.
XPS Pt4f spectra show a broadening and an asymmetry at
lower binding energy that, on the basis of a simple charge
potential model which neglects changes in final-state, has been
attributed to a component associated with Pt centers showing
an increase in the electron density. Considering the data analysis
of the Pt4f spectrum, the final state effects could be also
responsible of the spectral broadening after interaction, but these
might be accounted for only by appropriate calculations.
However, as will be shown in the following discussion about
XPS data analysis, the conclusion that Pt(II), after linkage to
sulfur-containing compounds, undergoes chemical shift toward
lower binding energy is supported by literature reports for
platinum complexes with ligands bound to the metal through
Pt-S bonds.31–34
The Pt4f spectral broadening induced to suppose the presence
of chemically different Pt sites. By means of a curve-fitting
analysis, each Pt4f spin-orbit component of the experimental
spectrum of samples 1-4 results from the combination of two
peaks as associated to two Pt atoms involved in different
chemical environments; the first Pt4f7/2 component is found at
nearly 72.3 eV for all samples, the second one at about 73.0
eV. The signal at higher BE occurs at the same energy value as
the Pt4f7/2 spin-orbit components observed in the pristine
samples, i.e. prior to H2S exposure. Curve fitting of Pt4f spectra
was performed by using four peaks generated by the two
spin-orbit components 4f7/2 and 4f5/2, separated by a spin orbit
coupling constant ∆ ) 3.3 eV (as reported for the pristine
samples 17) and with the expected area ratios 4f7/2:4f5/2 ) 4:3.
By comparing the Pt4f spectra as measured on pristine
samples and on samples exposed to H2S vapors (Figure 2), it is
Analysis of the Pt4f and S2p signals shed some light on the
chemical interaction arising between square-planar Pt(II) centers
and H2S molecules, that, as will be shown in the following, is

7370 J. Phys. Chem. A, Vol. 112, No. 32, 2008
Battocchio et al.
Figure 5. Difference of EXAFS signal in K-space before and after
H2S exposure for 1-4. The residue oscillation is attributed to Pt-S
bond formation.
Figure 4. Pt4f XPS spectra collected on samples 1-4 after exposure
to H2S gas.
noteworthy that for treated samples the signals become broad-
ened and that they show the presence of a small asymmetry,
arising by the new spectral component, at lower BE. Further-
more, for the higher energy Pt4f component both BE and fwhm
values, as taken from the Voigt profiles used for peak fitting,
are effectively completely similar to the ones observed for
pristine samples, as evidenced in Figure 2 for sample 4 and
summarized for all samples in the inserted table. On the basis
of the above-reported discussion, we assign this Pt signal to
the oligomer centers not involved in the reaction taking place
with H2S, then remaining unperturbed.
Conversely, the Pt4f7/2 spectral component that occurs at a
lower BE value (∼72.3 eV) can be associated with Pt metal
sites coordinating electron donor ligands, as for instance H2S;
this interpretation appears reasonable and is also in accord with
literature data.31,32 Among others, D. Atzei and co-workers
reported a Pt4f7/2 BE value of 72.5 eV for the PtBr2ttz2, PtCl2ttz2,
and [Pt3(ttz)8]C16 coordination compounds (ttz ) 1,3-thiazoli-
Figure 6. Tridimensional structure of the pentacoordinated Pt(II)
cluster. The sulfur atom is in the apical position, on the z axis; the two
P atoms and the two acetylene units are in the xy plane.
dine-2-thione), that was assigned to Pt(II) centers bonded to
sulfur.33,34 Conversely, the Pt4f7/2 BE value of the reaction
precursor K2PtCl4 was 73.0 eV. Thus, an energy shift of nearly
0.5 eV to lower BE values was observed for Pt(II) centers
subsequent to Pt-S binding; this finding is in excellent
agreement with our Pt4f data.
Support to the above discussion and assignment comes from
the analysis of the data relative to the H2S. We have in fact the
evidence that the S2p signal, as measured for all the samples,

H2S Interaction with Pt(II)-Containing Polymetallaynes
J. Phys. Chem. A, Vol. 112, No. 32, 2008 7371
of a single-crystal Pt surface; therefore, we think that the
molecular organization at the surface is probably responsible
for the partial Pt site involvement in the reaction. Not all the Pt
sites are suitably organized at the surface and therefore available
for the reaction; nevertheless, excluding an extended H2S
diffusion, all of the Pt sites within a depth of about 60 Å can
be detected by the XPS. We have evidence that, under the
reported experimental conditions, H2S-molecule interaction
undergoes saturation, meaning that all the Pt sites available for
the reaction have been involved. The Pt-H2S/Pt ) 1/3 ratio is
then most likely due to steric arrangement of the Pt poly yne
molecules.
EXAFS. Extended X-ray absorption fine structure measure-
ments were carried out at the Pt-LIII-edge on pellet samples
before and during exposure to H2S. The chemical cell in the
measurement chamber was filled with H2S up to a final pressure
of 500 mbar, and considering that the cell volume is about 340
cm3, an estimate of the gas concentration gives nearly 0.007
mol in the cell and therefore [H2S] ) 20 mmol. The interaction
taking place between Pt poly ynes and H2S, already demon-
strated by XPS data analysis, was successively confirmed by
the EXAFS results by means of comparison between data
collected on the pristine samples and measurements performed
in the same energy range on H2S-exposed samples. Difference
EXAFS signals, obtained by subtracting the signal in k space
achieved for the pristine poly ynes to the k signal of the cor-
responding sample as exposed to H2S, are reported in Figure 5
for samples 1, 2, 3, and, respectively, 4. The residual oscillation
pattern can be reasonably associated with the new Pt-S bond
formation and the subsequent reorganization of other bonds.
Figure 7. (a) EXAFS signal weighted with k3 for 1-4 and respective
best fits; EXAFS data have been collected on samples during exposure
to H2S. (b) Fourier transformation of the EXAFS signal in R space
and best fits for 1-4 exposed to H2S.
The theoretical model chosen for fitting the experimental
EXAFS data was selected with the aim to take into account the
fraction of Pt centers bonded to H2S, as already estimated by a
semiquantitative XPS analysis in 1/3 of the total Pt sites.
subsequent to H2S exposure, occurs at nearly 162.2 eV, a BE
value that is significantly different from the one expected for
the unperturbed or physisorbed H2S and that we associate to
H2S chemically interacting with platinum. Our data appear fully
consistent with those reported in literature for sulfur-containing
molecules interacting with metals through the formation of a
sulfur-metal chemical bond35 as well as for H2S molecules
chemisorbed on metals,5,36 therefore supporting our assignment.
S2p BE for H2S physisorbed on metals is expected to occur
around 164 eV,5,37,38 a signal which is completely absent in our
samples, neither in pellets or thin films.
On the basis of the so far discussed experimental data, we
hypothesized that a chemical interaction occurs between samples
1-4 and H2S molecules, involving the formation of a coordina-
tion bond between Pt and S. Additionally, as reported in the
last column of Table 2, the atomic ratio evaluated between the
Pt4f7/2 peak at lower BE (72.3 eV) and the S2p signals was
estimated as being 1:1 for all samples; this result strongly
supports our interpretation because it is in perfect agreement
with that expected for the above hypothesized S-Pt bond
formation.
Semiquantitative XPS analysis (see Table 2) leads to estima-
tion of the stoichiometry among platinum and adsorbed H2S.
Analysis of the data reported in the last column of Table 2 for
all samples shows that the low energy Pt4f7/2 component is
nearly 33% of the whole Pt4f7/2 intensity. This experimental
result indicates that under the chosen experimental conditions
for gas exposure (H2S pressure of about 500 mbar in the
chemical cell containing the sample pellet) both complex 1 and
oligomers 2, 3, and 4 exhibit the tendency to react with the
same amount of H2S.
As a consequence, for each examined compound the overall
Pt-LIII-edge EXAFS signal was ascribed as derived by 2/3
(nearly 66%) of square-planar Pt centers (the model used in
EXAFS data analysis of pristine samples) and 1/3 (33%
approximately) of pentacoordinated pyramidal Pt centers. The
pyramidal model was tentatively accounted for by adding a fifth
coordination position to the Pt square-planar model; the
theoretical Pt-S bond length was hypothesized as 2.53(1) Å,
as suggested by XRD measurements performed on similar
compounds and previously reported in the literature.7 A 3D
scheme of the square-pyramidal Pt(II) cluster, with the sulfur
atom in the apical position and the phosphine groups and
acetylene units lying in the xy plane, is shown in Figure 6.
The EXAFS signals ꢀ(k) weighted for k3 for samples 1-4 as
exposed to H2S molecules are shown in Figure 7a, whereas the
associated Fourier transforms are reported in Figure 7b, together
with the related best fits. The contribution to the total XAS signal
arising from the 66% tetracoordinated Pt centers plus the 33%
pentacoodinated transition metal from the atoms C1, P, C2, Cl
(in samples 1 and 2) and (for pentanuclear theoretical model P
only) S atoms was accounted for by including either the single,
double, or triple scattering paths of the photoelectron. In order
to describe the single scattering for Pt-C1, Pt-P, Pt-C2, Pt-Cl
(in samples 1 and 2) of the tetranuclear theoretical model (T),
both bond length and DW factors RT, σT, RT1, σT1, RT2, σT2,
RT3, σT3 were used. Conversely, in order to describe the single
scattering Pt-C1, Pt-P, Pt-C2, Pt-Cl (in samples 1 and 2),
and respectively Pt-S of the pentacoordinated (P) model, we
employed the bond length and DW factors RP, σP, RP1, σP1, RP2,
σP2, RP3, σP3, RP4, σP4. The multiple scattering paths
As a hypothesis for why only 1/3 of all Pt centers, as detected
by XPS, interact with H2S, we could say that this is not a case

7372 J. Phys. Chem. A, Vol. 112, No. 32, 2008
TABLE 3: EXAFS Data for 1-4 Exposed to H2S
Battocchio et al.
a
scattering path
1
2
3
4
single scattering Pt-C1
single scattering Pt-P
RT
RP
1.98(1)
1.99(1)
14(2)
2.33(8)
2.34(9)
21(7)
1.92(2)
1.96(1)
54(5)
2.31(2)
2.32(4)
49(1)
1.89(8)
1.99(8)
51(4)
2.24(1)
2.35(8)
46(2)
2.00(8)
2.06(3)
79(5)
2.31(2)
2.37(5)
55(2)
σ × 104
R1T
R1P
σ1 × 104
single scattering Pt-C2,
double scattering Pt-C2-C1,
triple scattering Pt-C1-C2-C1
R2T
3.20(1)
3.11(7)
3.06(8)
3.21(4)
R2P
3.21(7)
52(9)
2.28(7)
2.29(9)
21(7)
2.58(9)
43(1)
0.018
3.16(1)
111(3)
2.26(2)
2.28(5)
49(1)
2.65(3)
113(2)
0.020
3.23(1)
197(1)
-
-
-
2.59(9)
486(7)
0.020
3.30(2)
59(7)
-
-
-
2.61(8)
39(6)
0.014
σ2 × 104
single scattering Pt-Cl
R3T
R3P
σ3 × 104
single scattering Pt-S
R-factor
R4
σ4 × 104
a R is in angstoms; σ is in 10-4 Å2. The numbers in parentheses are the uncertainties in the interatomic distances and DW factors. For each
sample, the global amplitude factor S02 and the edge position ∆E0 were fixed to the best-fit values and are the same for all the considered
paths. The R-factor is a least-squared residual, showing the goodness of fit.
TABLE 4: Wavelengths of Maximum Absorption and
and the bond length (R) and DW factors estimated from best fit
FWHM of the UV-Visible Spectra Collected on 1-4 before
using ARTEMIS25 software with FEFF8.2026 calculations are
reported in Table 3, together with the evaluated R factors.
and after Exposure to H2S Gas
1
2
3
4
The hypothesis of the structure reported in Figure 6 can be
reasonably justified by coordination chemistry; however, strong
support comes from the attempts to fit the EXAFS data of the
H2S-treated samples without sulfur, i.e. with a square planar
model. This has been made, and the result was not consistent,
producing an R-factor of about 0.100, compared to the R-factor
in the range 0.010-0.020 found for the mixed 2/3 tetracoor-
dinated-1/3 pentacoordinated model, i.e. with 1/3 of the overall
Pt sites pentacoordinated with the sulfur in the apical position
on the Pt.
UV-Visible Absorption. Optical characterization of pristine
samples has been reported in a previous work;17 a sharp peak
at wavelengths falling in the range 358-375 nm was observed
for all the oligomers (number of repeat units 4-10 up to 50)
due to π-π* transitions of the organic moiety and associated
to the first singlet excited-state extending throughout the
macromolecule. As a consequence of the reaction with H2S, a
peak at 357 nm is expected to appear as suggested by the
literature;7 unfortunately, the absorption spectra of Pt poly ynes
are quite broad, and their maximum absorption wavelengths
values are too close to 357 nm to distinguish a proper shoulder.
Nevertheless, a fwhm increase was observed in UV-visible
spectra for H2S-containing samples with respect to the fwhm
values estimated for H2S-free samples spectra, as evidenced by
UV-visible data that are summarized in Table 4 and by the
superimposition of absorption spectra reported in Figure 8.
λmax (nm) pristine sample
λmax (nm) sample + H2S
fwhm (nm) pristine sample
fwhm (nm) sample + H2S
347.60 360.80 340.80 370.40
347.60 360.80 341.00 370.40
53.20
56.00
46.15
50.13
61.63
79.62
43.56
47.64
Pt-C2-C1-Pt and Pt-C1-C2-C1-Pt were also considered by
means of the parameters RT2, σT2 and RP2, σP2, respectively.
The overall amplitude was defined as a sum of the two T and
P amplitudes, and the ratio AmpT/AmpP ) 2/1 was verified to
give the best fit results, as suggested by XPS data analysis. Fits
of the EXAFS data were carried out in R space after Fourier
Transform of the k3 weighted EXAFS data in the range k )
3.5- 11.0 Å-1; the R range for all the fits was R ) 1.0-3.5 Å,
The spectra of pristine samples displayed π-π* absorption
bands red-shifted with respect to the organic monomeric
precursor DEBP (found at 290 nm), and the red shift increased
by increasing the number of repetitive units in linear oligomers,
i.e. going from sample 1 to samples 2 and 4. This effect, that
has been observed for analogous organometallic systems,17,37
is indicative of an electronic delocalization occurring through
the Pt centers, involving a contact between the π orbitals of
the organic conjugated spacer with the Pt 5d and 6p orbitals.39,40
Sample 3, in which the organic moieties are disposed in cis
Figure 8. UV-visible spectra collected on 1-4 before and after
exposure to H2S gas; in H2S-containing samples, a fwhm increase due
to the interaction between Pt-poly ynes and the hydrogen sulfide
molecule is observed.
configuration around the Pt centers, showed a λ
value
max
considerably lower than the tetranuclear all-trans oligomer 2.

H2S Interaction with Pt(II)-Containing Polymetallaynes
J. Phys. Chem. A, Vol. 112, No. 32, 2008 7373
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EXAFS measurements, for instance, by N2 gas fluxing or by
dissolving H2S by means of chloroform and dichloromethane.
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obtained under such severe experimental conditions. Actually,
the first objective of our work at this stage was to verify that
the main reason for the already assessed interaction between
Pt-poly ynes and sulfur-containing gases is the transition metal,
and that, conversely, the aromatic moieties of the organometallic
macromolecules do not play a key role in the process. In the
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amounts of gas at lower pressure, in order to check the saturation
conditions and test the reversibility of the interaction.
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UV-visible absorption spectra have also been collected, and a
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Conclusions
The interaction of Pt-containing organometallic poly ynes
with H2S molecules has been investigated for one complex and
three oligomers differentiated in both chain length (4, 9 repetitive
units) and conformation (cis or trans). Several techniques, i.e.
XPS, EXAFS, and UV-visible absorption spectroscopies, have
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H2S to the Pt systems. Our plan considers further investigations
under more controlled exposure conditions. As a result of our
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Acknowledgment. We gratefully acknowledge all the GIL-
DA beamline staff for the support during measurements. We
also thank Prof. A. Martorana and Dr. F. Giannici of the
University of Palermo and Dr. A. Longo of the ISMN of the
CNR for lending us the chemical cell.
References and Notes
(1) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd
ed.; Dekker: New York, 1991.
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