Synthesis of triarylphosphines having para -SH and -SMe groups. Preparation of their complexes and formation of a monolayer on a gold surface

Article abstract of DOI:10.1016/j.jorganchem.2004.08.031

The phosphines P(C₆H₄-4-SR)₃ (R = H, Me, 2-C₅H₉O) and (C₆H₄ -4-SR)₂PCH₂CH₂P(C₆ H₄-4-SR)₂ (R = H, Me) have been synthesized. The phosphines with -SMe groups can be prepared by reaction of 4-BrC₆H₄SMe with either BuLi or magnesium (to generate the corresponding Grignard compound) followed by reaction with PCl₃ or Cl₂PCH₂ CH₂PCl₂, respectively. The methyl group can be eliminated by reaction with sodium in liquid NH₃. Other methods of protection/deprotection of the thiol group failed to afford the desired compounds. Reaction of 4-BrC₆ H₄SH with dihydropyrane afforded the protected thiol 4-BrC₆H₄S-2-C₅H₉O from which the corresponding phosphine was successfully synthesized. However, attempts to remove the tetrahydropyranyl group by reaction with AgNO₃-HCl, gave an insoluble polymer as product. Reaction of P(C₆H₄SR)₃ (R = H, Me) with Ni(CO)₄ affords the corresponding mono phosphine complex quantitatively. The complex with the unprotected thiol group can be absorbed on a gold surface and the corresponding ν_CO bands were detected by grazing angle Fourier transform infrared reflection absorption spectroscopy (grazing angle FTIR-RAS). Reaction of Rh(acac)(CO)₂ with P(C₆H₄SR)₃ (R = Me) affords the complex Rh(acac)(CO)(P(C₆H₄SR)₃) (R = Me), but if R = H a polymer insoluble in any solvent was obtained. The same occurs in the case of PtCl₂(CO)(DMSO). Apparently, once P(C₆H₄SH)₃ is coordinated to a metal not in the zero oxidation state, oxidation of the thiol group to disulphide becomes very easy even in a dinitrogen atmosphere.

Full text of DOI:10.1016/j.jorganchem.2004.08.031

Journal of Organometallic Chemistry 689 (2004) 3621–3630
www.elsevier.com/locate/jorganchem
Synthesis of triarylphosphines having para –SH and –SMe groups.
Preparation of their complexes and formation of a monolayer on a
gold surface
*
Fabio Ragaini , Luca Lunardi, Diego Tomasoni, Vittoria Guglielmi
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, ISTM-CNR, UniversitaÕ degli Studi di Milano,
via Venezian 21, 20133 Milano, Italy
Received 10 May 2004; accepted 19 August 2004
Available online 15 September 2004
Abstract
The phosphines P(C6H4-4-SR)3 (R = H, Me, 2-C5H9O) and (C6H4-4-SR)2PCH2CH2P(C6H4-4-SR)2 (R = H, Me) have been syn-
thesized. The phosphines with –SMe groups can be prepared by reaction of 4-BrC6H4SMe with either BuLi or magnesium (to gen-
erate the corresponding Grignard compound) followed by reaction with PCl3 or Cl2PCH2CH2PCl2, respectively. The methyl group
can be eliminated by reaction with sodium in liquid NH3. Other methods of protection/deprotection of the thiol group failed to
afford the desired compounds. Reaction of 4-BrC6H4SH with dihydropyrane afforded the protected thiol 4-BrC6H4S-2-C5H9O from
which the corresponding phosphine was successfully synthesized. However, attempts to remove the tetrahydropyranyl group by
reaction with AgNO3–HCl, gave an insoluble polymer as product. Reaction of P(C6H4SR)3 (R = H, Me) with Ni(CO)4 affords
the corresponding mono phosphine complex quantitatively. The complex with the unprotected thiol group can be absorbed on a
gold surface and the corresponding mCO bands were detected by grazing angle Fourier transform infrared reflection absorption
spectroscopy (grazing angle FTIR-RAS). Reaction of Rh(acac)(CO)2 with P(C6H4SR)3 (R = Me) affords the complex Rh(acac)
(CO)(P(C6H4SR)3) (R = Me), but if R = H a polymer insoluble in any solvent was obtained. The same occurs in the case of
PtCl2(CO)(DMSO). Apparently, once P(C6H4SH)3 is coordinated to a metal not in the zero oxidation state, oxidation of the thiol
group to disulphide becomes very easy even in a dinitrogen atmosphere.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Phosphines; Thiols; Gold; Monolayers; Transition metal complexes
1. Introduction
have been devoted to the electrochemical behavior of
gold surfaces modified by thiol-terminated organic
molecules [1]. In a few cases, metal complexes were
linked to a gold surface by means of a thiol group,
the anchoring ligand being for example 4-thiopyridine
[2] or a porphyrin bearing a thiol-terminated side
chain [3]. Although most catalytic studies are related
to the influence of the monolayer on electrochemical
reactions, the specific organization brought by a
monolayer can by itself modify the chemical reactivity
of a bound catalyst, as has been shown in at least one
case by Milstein and co-workers [4].
Studies of monolayers on different substrates are an
important research field. Among the most important
features of monolayers is the modification they induce
on both the chemical and electrochemical reactivity of
the surface on which they are deposited. Many studies
*
Corresponding author. Tel.: +39 2 5031 4373; fax: +39 2 5031
4405.
E-mail address: fabio.ragaini@unimi.it (F. Ragaini).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.08.031

3622
F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
Monolayers of catalytically active metal complexes
phine has not been described in the literature
previously. Its 31P NMR spectrum was reported in a
paper [10] and the compound was said to have been
prepared by a Grignard reaction, but no detail was
given.
Tri-(4-methylthio)phosphine (1) was synthesized by
treating 4-MeSC6H4Br with BuLi, followed by reaction
with PCl3, similarly to what reported in the literature for
its ortho analogue [6].
on a metal surface may display interesting properties,
because, apart from the specific organization of the
complexes on the surface, the bulk metal may act as
an electron reservoir, smoothing energy differences
during the catalytic cycle and thus accelerating the
reaction. In order for electronic effects of this kind
to be possibly observable, it is expected that the dis-
tance between the metal surface and the metal com-
plex should be short and the ‘‘linker’’ be
a
Br
1) BuLi 0 ˚C
P
conjugate molecule. The long alkyl chains commonly
employed as spacers are expected to strongly decrease
the electronic communication between the metal sur-
face and the complex. In this light, the previously re-
ported monolayers based on 4-thiopyridine as linker
are good candidates for further studies, but it is also
clear that most homogeneous catalysts reported to
date employ phosphines as ligands. The mode of
absorption of a thiol-containing phosphine on a gold
surface is not obvious, because it is well known that
phosphines can directly interact with metallic gold
through the phosphorus atom, independent of the
presence of a thiol group [5]. Several phosphines hav-
ing a pendant thiol group have been synthesized to be
employed as hemilabile ligands, including tris(2-thio-
phenyl)phosphine [6] and some of its complexes [7].
However, the thiol group in these molecules is most
often not in the correct position to bind to a metal
surface and usually chelates to the metal. A few phos-
phines having the sulfur atom remote from the phos-
phorus one have been reported [8] but their behavior
with respect to a metal surface was not reported. To
the best of our knowledge, in only one case was a
thiol-containing phosphine, Ph2PCH2CH2SH, an-
chored on a metal surface up to now [9]. The possible
interaction of the phosphorus atom with the metal
surface was not discussed and reaction of the ab-
sorbed phosphine with Rh2(l-Cl)2(CO)4 afforded sev-
eral compounds depending on the reagents
concentrations.
2) PCl3 0 → 20 ˚C
MeS
SMe
3) HCl 0.2 M
SMe
SMe
1
ð1Þ
The crude product can be recrystallized to afford pure
1, in a 40.7% isolated yield. The 31P NMR spectrum of 1
shows a single peak consistent with the reported value
[10].
2.2. Deprotection of 1 and synthesis of tri-(4-thiophe-
nyl)phosphine (2)
In the literature, it has been reported that deprotec-
tion of a methylthio group to afford the corresponding
thiol can be achieved by reaction of the former with so-
dium t-butylthiolate in DMF, followed by aqueous HCl
[11]. Application of the published protocol to 1, how-
ever, afforded a mixture of products in which, according
to mass spectral and NMR data, the predominant prod-
uct is the deprotected phosphinsulfide (4-HSC6H4)3P@S
(Scheme 1). Only trace amounts of 2 were present and
attempts to eliminate the undesired sulfur atom from
the phosphinsulfide failed.
Transformation of 1 into 2 could be effectively
achieved by the use of sodium in liquid ammonia, fol-
lowed by treatment with aqueous NH4Cl (Eq. (2))
[12,13]. Pure 2 was isolated in a 87% yield, provided care
is taken to exclude air from the manipulation of the
reaction solution even at the neutralization step with
NH4Cl. Failure to do so sharply decreased the yield to
a mere 11%.
In this paper, we report the synthesis of new phos-
phines having –SH or –SMe groups and of some of their
complexes, and the successful formation of a monolayer
of one of them on a gold surface.
S
P
2. Results and discussion
SH
SH
HS
HS
1)tBuSNa, DMF
160˚C
2) HCl (3M)
SMe
P
2.1. Synthesis of tri-(4-methylthio)phosphine (1)
SH
P
MeS
Since initial attempts to prepare P(4-HSC6H4)3 by
reaction of 4-HSC6H4Br with BuLi, followed by reac-
tion with PCl3 afforded a mixture of products, we
decided to first synthesize the methyl protected ana-
logue P(4-MeSC6H4)3 (1). The synthesis of this phos-
1
SMe
2
SH
Scheme 1.

F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
3623
P
Br
P
1) Na, NH3(liq.)
SMe
2) NH4Cl
Br
Br
MeS
HS
SH
AgNO3, H2O
MeOH / H2O
HCl
+
HO
SMe
1
- AgCl
SH
2
O
O
S
SH
SAg
ð2Þ
3
Scheme 2.
2.3. Other protection strategies and synthesis of (4-tetra-
hydropyranylthiophenyl)phosphine (4)
olate group. Application of this protocol to 3 gave excel-
lent results and 4-bromothiophenol was regenerated in
almost quantitative yields (Scheme 2).
Reaction of 3 with BuLi and PCl3 following the same
protocol employed for the synthesis of 2 afforded a mix-
ture of difficult to separate products. However, generat-
ing the Grignard reagent instead of the lithiated arene
afforded sufficient amounts of the phosphine 4 to allow
purification by column chromatography (Eq. (4), 12%
isolated yields of the pure product).
In order to find a more user friendly synthetic method
which may allow not to use liquid ammonia, other pro-
tection–deprotection strategies were investigated.
Protection of the thiol group with the –SiMe3
group was first performed, but the same rearrange-
ment problems reported in the literature [14] were ob-
served upon treatment of the protected product with
butyllithium.
Another protection strategy for the –SH group re-
ported in the literature consists in the reaction with
dihydropyrane to afford the tetrahydropyranil pro-
tected thiol. When we applied the reaction protocol
described for thiophenol [15] to 4-bromo-thiophenol,
the protected derivative 3 was obtained in only a
15% isolated yield. The main product was 4-Br-
C6H4S–CH2-SC6H4Br-4, as identified by GC–MS
analysis. Since the reaction protocol requires treating
the solution after reaction with NaOH, followed by
extraction with CH2Cl2, the observed byproduct
Br
P
1) Mg/THF
O
2) PCl3, 0 ˚C
S
S
3) HCl 0.2 M
O
S
O
S
O
3
4
ð4Þ
However, when deprotection was attempted by the
clearly derives from
a condensation reaction of
same protocol employed for the parent bromide
(Scheme 2), a solid yellow product was obtained upon
treatment with AgNO3, which did not redissolve upon
addition of HCl and was completely insoluble in any
solvent. The formation of this apparently polymeric
compound is related to the one of similar materials ob-
tained during attempts to generate some complexes of
2 and will be further discussed later. In any case, this
unwanted reaction prevents the synthetic route via 4 to
be employed for the synthesis of 2 and the strategy via
1 remains the only viable at the moment.
CH2Cl2 with the thiolate generated from unreacted
bromothiol. Simply by refluxing the solution for sev-
eral hours, instead of working at room temperature,
increased the yield to 86%.
Br
Br
[p-CH3C6H4SO3][PyH]
ð3Þ
+
CH2Cl2 40 ˚C
O
S
O
SH
3
2.4. Synthesis of bis(di-(4-thiophenyl)phosphino)ethane
(6)
Before proceeding to the synthesis of the phosphine,
the deprotection strategy was investigated. Despite some
compounds of this kind have been reported to be sensi-
tive to acidic hydrolysis [16], 3 was not hydrolyzed to
any detectable extent even by hot concentrated HCl. It
has been reported that tetrahydropyrane protected thi-
ols resistant to acidic hydrolysis can be deprotected in
a two steps procedure by treating them with AgNO3,
followed by HCl [15,17]. During the first step, a silver
thiolate is formed, which precipitates out the solution
and is collected by filtration. In the second step, HCl
precipitates silver as its chloride and protonates the thi-
The same synthetic strategy described in Eqs. (1) and
(2) was next applied to the synthesis of a chelating phos-
phine, by employing Cl2PCH2CH2PCl2 in place of PCl3
(Scheme 3).
1
The aromatic region of the phosphorus-coupled H
NMR spectrum of both 5 and 6 was quite complex,
but simulation of the spectrum of 6 allowed the com-
plete identification of all coupling constants. The signal
for the central CH2 groups was expected to be a doublet
of doublets, but is observed as a triplet because of the
almost coincident JP–H coupling constants, despite the

3624
F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
1) BuLi 0 ˚C
Br
P
P
SMe
MeS
2) Cl2PCH2CH2PCl2
0
20 ˚C
3) HCl 0.2 M
5
SMe
SMe
MeS
1) Na, NH3(liq.)
2) NH4Cl
P
P
HS
SH
6
HS
Scheme 3.
SH
fact that the two phosphorus atoms are not equivalent
with respect to each CH2 group.
In search for more oxidation stable complexes, we
investigated the complexes Rh(acac)(CO)(L) (L = 1 or
2, acacH = acetylacetone). Several phosphine complexes
of this kind are known and were obtained by reaction of
Rh(acac)(CO)2 with the corresponding phosphine [19].
The starting Rh(acac)(CO)2 was first reported by Bonati
and Wilkinson [20], but the original synthesis is quite
long. A much more straightforward synthesis has been
reported later, which employs dimethylformamide as
both the solvent and the reducing agent (Eq. (6)) [21].
2.5. Synthesis of complexes of 1 and 2
Interaction of 2 with excess Ni(CO)4 in CH2Cl2 at
room temperature generates the complex Ni(CO)3(P
(4-C6H4SH)3) (7), which can be isolated in a pure form
by evaporating the solvent and excess Ni(CO)4 in va-
cuo (Eq. (5)).
RhCl3 ꢁ 3H2O þ acacH þ nDMF
O
O
O
C
C
C
! RhðacacÞðCOÞ
ð6Þ
CH2Cl2
RT
2
P
Ni
Ni(CO)4
+
P
We succeeded in reproducing the reaction in Eq. (6),
although in our hands the yield was quite low (23%).
Performing the reaction under a CO atmosphere did
not improve the yield.
Reaction of Rh(acac)(CO)2 with 1 in CH2Cl2 cleanly
afforded Rh(acac)(CO)(P(4-C6H4SMe)3) (8), as indi-
cated by IR and elemental analysis.
- CO
HS
SH
HS
SH
2
SH
SH
7
ð5Þ
The IR spectrum of 7 in CH2Cl2 shows the expected
two absorptions at 2066.4 (m) and 1993.2 (vs) cmꢀ1
.
P
The first value is slightly lower than that reported by Tol-
man in his famous review [18] for the analogous complex
with triphenylphosphine (2068.9 cmꢀ1) and is intermedi-
ate between the values reported for tri-(p-tolyl)phosphine
CH2Cl2
Rh(acac)(CO)2 +
MeS
SMe
RT
1
CH3
O
H
C
SMe
(2066.7 cmꢀ1
) and tri-(p-methoxyphenyl)phosphine
(2066.1 cmꢀ1) complexes. This indicates that the thiol
group in 6 has a slight donating character.
H3C
O
C
O
Rh
P
Complex 7 is extremely air sensitive. Handling its
solutions with a standard nitrogen line and Schlenk
apparatus was not sufficient to prevent partial decompo-
sition within a few hours to afford an insoluble species
showing in the IR spectrum (in nujol) absorptions at
2067 (m), 2000 (s), and 1943 (s) cmꢀ1. This decomposi-
tion is extremely noxious from the point of view of the
characterization of monolayers (see next paragraph),
since two of the absorptions are almost coincident with
the ones of 7.
MeS
SMe
8
SMe
ð7Þ
However, when the same reaction was repeated by
employing 2, an insoluble product, 9, was immediately
formed, which did not dissolve in any solvent. Both

F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
3625
complexes showed in the IR spectrum a single absorp-
tion for the CO group at 1975 cmꢀ1 for 8 (in CH2Cl2)
and 1979 cmꢀ1 for 9 (in nujol), in complete agreement
with what expected from the spectra of other
Rh(acac)(CO)(phosphine) complexes [19].
been noted by others that the mica surface is very flat
on a small domain level (such as that investigated by
STM), but is quite rough on the much larger extension
required to record an IR spectrum in the reflection–ab-
sorption mode [23].
The same problem encountered with 9 was observed
when PtCl2(DMSO)2 was treated with 2 under a CO
atmosphere. An insoluble product, 10, immediately
formed again, despite the fact that the analogous reac-
tion with triphenylphosphine cleanly afforded the
known and soluble PtCl2(CO)(PPh3) [22]. The IR spec-
trum of 10 (in nujol) shows a single absorption in the
carbonyl region at 2103 cmꢀ1, close to the one observed
by us for the triphenylphosphine complex (2110 cmꢀ1, in
CH2Cl2).
The complete insolubility of 9 and 10 seems to indi-
cate a polymeric nature. However, the similarity of their
IR spectra with those of monomeric compounds of
known composition strongly suggests that the coordina-
tion sphere of the metal has not been involved in the
polymerization. Given the high tendency of thiol groups
to be oxidized to disulphide and the presence of three
such groups on each complex molecule, the best expla-
nation for the observed behavior is that oxidation of
at least two thiol groups per molecule leads to disul-
phide-linked polymeric products. The role of the metal
in this process is however unclear and, to the best of
our knowledge, unprecedented. Since the reactions are
very fast even under a dinitrogen atmosphere and with
freshly purified solvents, even the nature of the oxidant
is obscure and it cannot be excluded that other pathways
for the polymerization (e.g. one involving other posi-
tions of the phosphine aryl rings) are operating.
When the gold lamina was immersed for 5 min in a
solution of Ni(CO)3(P(4-C6H4SH)3) (7) in THF, rinsed
with more THF in a nitrogen stream and immediately
placed in the nitrogen purged sample compartment of
the IR spectrophotometer, the IR spectrum showed
the presence of two absorptions at 2070 and 2001
cmꢀ1 (Fig. 1(b)), close to the positions of the bands of
7 in CH2Cl2 solution (Fig. 1(a)). The spectra were
recorded with the grazing angle FTIR-RAS technique.
This technique has already been used successfully for
the characterization of monolayers on gold [24]. It is
important to note that the degeneracy of the lower fre-
quency carbonyl stretching of the Ni(CO)3 moiety is
not removed by the coordination to the gold surface.
This indicates that the complex is interacting with the
(a)
2100
2050
2000
1950
1900
(b)
2.6. Interaction of phosphine complexes with a gold
surface
To study the absorption of metal complexes on a me-
tal surface, a gold polycrystalline lamina was made
mechanically flat by polishing it with abrasive paper of
decreasing grain (up to n 2000). The surface was then
further smoothed with alumina powder of decreasing
size (1, 0.3, 0.03 lm). Between every polishing cycle with
alumina and after the last, the lamina was irradiated
with ultrasounds for 10 min in an ultrasound bath to re-
move any residue of the alumina employed. Chemical
cleaning was then effected by immersing the lamina in
a 1:1 vol/vol concentrated H2SO4/35% H2O2 solution
for 30 min, followed by extensive washings with HPLC
grade water and methanol. When the lamina was reused,
only the treatment with 0.03 lm alumina and the follow-
ing chemical cleanings were performed. Some experi-
ments were also performed with gold surfaces obtained
by sputtering a gold layer on freshly cut mica surfaces,
but the IR spectra recorded were invariably of poorer
quality and will not be discussed here. It has already
2100
2050
2000
1950
1900
(c)
2100
2050
2000
ν(CO)/cm-1
1950
1900
Fig. 1. (a) IR absorption spectrum of Ni(CO)3(P(4-C6H4SH)3) (7) in
CH2Cl2. (b) Grazing angle FTIR-RAS spectrum of a gold lamina
which had been immersed in a solution of 7 for 5 min. (c) Grazing
angle FTIR-RAS spectrum of a gold lamina which had been immersed
in a solution of 7 for 4 h.

3626
F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
surface by all the three thiolate groups. If only one or
two of the sulfur atoms were interacting with gold, the
C3v symmetry axis of the Ni(CO)3 moiety would be
tilted with respect to the metal surface and splitting of
the lower frequency band should be observed. Due to
the very small amount of complex deposited on the sur-
face, it must be acknowledged that if a small fraction of
the complex molecules were interacting with the gold
lamina through only one or two sulfur atoms, the corre-
sponding IR absorptions may not be distinguishable
from the background noise. However, in case this type
of coordination is indeed occurring, it appears to be
the exception rather than the rule.
To the best of our knowledge only one precedent,
mentioned in Section 1 [9], has been reported in the
literature for a monolayer of a phosphine metal complex
on a metal surface, although metal phosphine complexes
anchored to oxide surfaces are quite common. The
known tendency of phosphines to interact with bulk met-
als through the phosphorus atom has surely been a con-
ceptual and experimental obstacle to this goal, but the
successful formation of a monolayer of 7 on a gold sur-
face indicates that preforming the complex can be an
effective way of circumventing the problem.
first and then react the functionalized lamina with
Rh(CO)2(acac), but with no success. As previously men-
tioned, phosphines can interact with gold even through
the phosphorus atom. Unfortunately, IR absorptions
due to residual water vapor in the instrument chamber
prevented us from characterizing the phosphine-only
monolayer in a reliable way to asses if phosphorus absorp-
tion is indeed the cause for this failure.
3. Conclusions
In this paper, we have reported the synthesis of the
new phosphines 1, 2, 5, and 6, having –SH or –SMe
groups in the para positions of the aryl rings, and of
three of their complexes. An unexpected easy polymeri-
zation was observed when the thiol-containing phos-
phine 2 was contacted with a rhodium(I) or a
platinum(II) complex and the same apparently occurs
in the presence of silver(I) salts. This polymerization
strongly limits the range of metal fragments that can
give stable, soluble, complexes with these phosphines.
The nickel complex Ni(CO)3(P(4-C6H4SH)3) (7) could
be successfully anchored onto a gold surface to yield a
monolayer. The latter was unstable, likely due to the
high oxygen sensitivity of the complex. Based on the
accumulated experience, a more stable monolayer will
probably require the preparation of an air-stable com-
plex of a metal in a zero oxidation state. Attempts in this
direction will be made in the future.
Unfortunately, the afore mentioned absorptions dis-
appeared in a few minutes. If the lamina was kept in
the solution of 7 for several hours, a more persistent
spectrum was obtained (Fig. 1(c)), which also shows
an additional band at 1943 cmꢀ1. However, as previ-
ously mentioned, this band indicates that 7 has been oxi-
dized and even the two other bands must be ascribed to
the decomposition product and not to 7.
4. Experimental
Although disappointing, it should be noted that insta-
bility of thiol monolayers on gold is very common. Pyridi-
nethiolate monolayers start to decompose after a few
minutes in solution [25] and other short chain thiol mono-
layersalso do the same [25,26]. The monolayersformed by
pyridinethiolate complexes are somewhat more stable,
but they also decompose with time [2]. The decomposition
has been identified as arising from S–C bond breaking,
with formation of surface oligomeric sulfur species
[25,26]. The instability of the monolayer of 7 may be
due to the same kind of process. However, given the high
sensitivity of 7 to oxygen and the fact that exposure of the
gold lamina to air is unavoidable when mounting it on the
sample holder of the IR spectrophotometer, we consider it
also likely that oxidation of the complex is responsible for
the decomposition of the monolayer.
Complexes 9 and 10 were too insoluble and no IR
absorptions were detected on the gold lamina after
immersion in a THF suspension of these complexes. Even
8, having –SMe groups on the phosphine, did not absorb
onto the gold surface, supporting the conclusion that
interaction of 7 with gold must be driven by a reaction
of the –SH bonds and is not just a physical absorption.
An attempt was also made to absorb the free phosphine
4.1. General procedure
Unless otherwise specified, all reactions and manip-
ulations were performed under a N2 atmosphere using
standard Schlenk apparatus and magnetic stirring. All
glassware was kept in an oven at 125 ꢁC for at least 2
h and evacuated while still hot before use. Solvents
were dried and distilled by standard procedures and
stored under dinitrogen. Rh(CO)2(acac) [21] and
PtCl2(DMSO)2 [27] were synthesized by methods re-
ported in the literature. All other starting materials
were commercial products and were used as received.
NMR spectra were recorded on a Bruker AC 300
FT (working at 300 MHz for 1H NMR spectra and
at 121 MHz for 31P NMR spectra) at RT. Simulation
of the 1H NMR spectrum of 6 was performed with
the gNMR 4 program. Transmission IR spectra were
recorded on a Bio Rad FTS-7 FT-IR. Grazing angle
FTIR-RAS spectra were recorded on a Digilab FTS-
40 FT-IR spectrophotometers, equipped with a Specac
variable angle specular reflectance accessory. Elemen-
tal analyses and mass spectra were recorded in the
analytical laboratories of Milan University.

F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
3627
4.2. Synthesis of P(4-C6H4SMe)3 (1)
CDCl3, 298 K): d = 7.23 (dd, Jortho = 8.4, JH–P = 1.5
Hz, 6H, Hmeta
Hz, 6H, Hortho
(121 MHz, 298 K, CDCl3): d = ꢀ7.39.
P), 7.14 (dd, Jortho = 8.4, JP–H = 7.4
P), 3.50 (s, 3H, SH). 31P NMR
to
To a 100 ml Schlenk flask were added 4-bromothioan-
isole (5.77 g, 28.4 mmol) and diethyl ether (30 ml). To the
obtained solution was added during 2 h BuLi (18 ml of a
1.6 M solution in hexanes, 28.8 mmol). During the addi-
tion, the temperature was maintained at 0 ꢁC by an ice
bath. After the addition was over, the solution was stir-
red at the same temperature for an additional hour and
a PCl3 (1.31 g, 9.56 mmol) solution in diethyl ether (20
ml) was slowly added at 0 ꢁC in 2.5 h. A yellow precipi-
tate formed during this addition. When the addition was
over, 0.2 M HCl (18.8 ml) was added at room tempera-
ture. The yellow solid immediately dissolved upon addi-
tion of HCl and a colorless one formed instead. After
stirring the suspension at room temperature overnight,
the solid was collected by filtration in the air, washed
with water (3 · 30 ml), methanol (3 · 10 ml), and diiso-
propyl ether (3 · 10 ml). Recrystallization from n-buta-
nol (35 ml) afforded the analytically pure product. 1.54
g, 40.7% yield. Anal. Found: C, 62.99; H, 5.20. Calc.
to
4.4. Synthesis of (4-C6H4SMe)2PCH2CH2P(4-C6H4-
SMe)2 (5)
The synthesis was performed as described for 1,
employing the following reagents amounts: 4-brom-
othioanisole (4.071 g, 20.04 mmol), diethyl ether (25
ml), BuLi (13.75 ml of a 1.6 M solution in hexanes, 22
mmol), Cl2PCH2CH2PCl2 (811.3 mg, 3.5 mmol) dis-
solved in diethyl ether (10 ml), 0.2 M HCl (14 ml).
The product was recrystallized twice from n-butanol.
579.6 mg, 28.5% yield. Anal. Found: C, 61.65; H, 5.47.
Calc. for C30H32P2S4: C, 61.83; H, 5.53. {31P}1H
NMR (300 MHz, CDCl3, 298 K): d = 7.23 (d, J = 8.5
Hz, 8H, Ar–H), 7.17 (d, J = 8.5 Hz, 8H, Ar–H), 2.49
(s, 12H, CH3), 2.03 (s, 4H, CH2). 31P NMR (121
MHz, CDCl3, 298 K): d = ꢀ13.98, m.p. 143–146 ꢁC.
1
for C21H21PS3: C, 62.97; H, 5.28. H NMR (300 MHz,
CDCl3, 298 K): d = 7.21–7.22 (m, 12H, Ar–H), 2.49 (s,
9H, CH3). 31P NMR (121 MHz, 298 K, CDCl3):
d = ꢀ7.55, m.p. 135–137 ꢁC.
4.5. Synthesis of (4-C6H4SH)2PCH2CH2P(4-C6H4-
SH)2 (6)
The synthesis was performed as described for 2,
employing the following reagents amounts: (4-
MeSC6H4)2PCH2CH2P(4-C6H4SMe)2 (5) (410.6 mg,
0.706 mmol), NH4Cl (302 mg, 5.65 mmol), NaOH 1 M
(50 ml). 263.8 mg, 71.0% yield. Anal. Found: C, 59.32;
4.3. Synthesis of P(4-C6H4SH)3 (2)
A three necked flask was connected to a dinitrogen
line and a NH3 line, leaving the third neck free for the
additions. In a dinitrogen atmosphere was added 1
(1.00 g, 2.5 mmol). The flask was cooled down to
ꢀ78 ꢁC by a dry ice-acetone bath and about 20 ml
of liquid NH3 were condensed into the flask. Small
pieces of metallic sodium were then added in about 1
h, until a blue color persisted for 15 min. During this
time, it was necessary to add more NH3 to compensate
for the evaporated one. The blue suspension was stir-
red for an additional 30 min, after which solid NH4Cl
was added until a clear solution was obtained, which
was left at room temperature up to complete evapora-
tion of NH3. The so obtained solid was treated with a
degassed 1 M NaOH aqueous solution (100 ml). The
solid dissolved almost completely. The solution was fil-
tered through filter paper mounted on a Teflon cann-
ula and taken to pH 3 by the addition of a degassed
HCl solution (20 ml 37% HCl and 40 ml H2O). The
precipitated solid was separated by filtration, washed
with water and dried in vacuo. Then it was dissolved
in diethyl ether (a small amount of impurities re-
mained undissolved) and the solution was dried with
Na2SO4. After separating sodium sulfate and the
impurities by filtration, the solution was evaporated
to afford the analytically pure product. 780 mg, 87%
yield. Anal. Found: C, 59.94, H, 4.21. Calc. for
C18H15PS3: C, 60.33; H, 4.22. 1H NMR (300 MHz,
1
H, 4.70. Calc. for C26H24P2S4: C, 59.30; H, 4.59. H
NMR (300 MHz, CDCl3, 298 K, J values resulting from
{31P}1H NMR spectrum and computer simulation):
d = 7.21 (m, Jortho = 8.4, JH–P = 0.6 Hz, 8H, Hmeta), 7.16
(m, Jortho = 8.4, JH–P = 4.1 Hz, 8H, Hortho), 3.48 (s, 4H,
SH), 1.98 (pt, JHa–P = 4.4, JHb–P = 4.2 Hz, 4H, CH2).
31P NMR (121 MHz, CDCl3, 298 K): d = ꢀ13.88.
4.6. Synthesis of S-(2-tetrahydropyranil)-4-bromothio-
phenol (3)
The procedure is adapted from the one reported in
the literature for thiophenol [15]. To a 50 ml Schlenk
flask were added 4-bromothioanisole (4.52 g, 23.9
mmol), pyridinium p-toluensulfonate (597 mg, 2.34
mmol), tetrahydropyrane (previously distilled over so-
dium and stored under N2, 3.5 ml, 38.4 mmol), and
CH2Cl2 (12 ml). The solution was stirred at room tem-
perature for 2 h, then refluxed for 4 h. After cooling,
it was extracted with 10% aqueous NaOH (4 · 20 ml)
and saturated aqueous NaCl (2 · 20 ml). The organic
phase was dried with Na2SO4 and the solvent evapo-
rated in vacuo. The yellow oil was analyzed by
GC–MS and found to contain the desired product, con-
taminated by small amounts (ꢂ4% overall) of the start-
ing thiol and of the disulphide 4-BrC6H4SSC6H4Br-4.

3628
F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
No further purification was attempted and the product
was employed as such in the following reaction.
{31P}1H NMR (300 MHz, CDCl3, 298 K): d = 7.42
(d, J = 7.6 Hz, 2H, H7 and H9), 7.19 (d, J = 7.6 Hz,
2H, H6 and H8), 5.28 (m, 1H, H1), 4.15–4.19 (m, 1H,
H5 or H5 ), 3.59–3.63 (m, 1H, H5 or H5), 2.01–2.06
(m, 1H), 1.81–1.88 (m, 2H), 1.62–1.68 (m, 3H). 31P
NMR (121 MHz, CDCl3, 298 K): d = ꢀ6.74.
0
0
Br
7 H
6 H
H7'
H6'
O
H1
H5'
H5
S
4.8. Synthesis of Ni(CO)3(P(4-C6H4SH)3) (7)
2 H
H4
H4'
To a Schlenk tube were added P(4-C6H4SH)3 (2) (100
mg, 0.28 mmol) and CH2Cl2 (5 ml) and the tube was
cooled to 0 ꢁC. Another Schlenk tube under dinitrogen
was connected to a Ni(CO)4 bottle and cooled to ꢀ78
ꢁC by a dry ice-acetone bath. About 0.3 ml of Ni(CO)4
were condensed into the tube. From this tube, liquid
Ni(CO)4 (100 lL, 136 mg, 0.80 mmol) was withdrawn
with a microsyringe and added to the Schlenk tube con-
taining the phosphine. The solution was stirred at 0 ꢁC
for a few minutes and then brought to RT. At this stage,
an IR spectrum of the solution showed the presence of
absorptions at 2066 (m), 1993 (vs) (mCO), and 1579,
2' H
H
H
3
3'
1H NMR (300 MHz, CDCl3, 298 K): d = 7.42 (d,
0
J = 8.6 Hz,0 2H, H7 and H7 ), 7.35 (d, J = 8.6 Hz, 2H,
H6 and H6 ), 5.20 (dd, J1 = 5.5, J2 = 3.9 Hz, 1H, H1),
0
0
4.14–4.19 (m, 1H, H5 or H5 ), 3.58–3.62 (m, 1H, H5
or H5), 1.88–2.05 (m, 1H), 1.80–1.87 (m, 2H), 1.62–
1.65 (m, 3H).
4.7. Synthesis of tri-(S-(2-tetrahydropyranil)-4-thiophe-
nyl)phosphine (4)
1548, 1480, 1390, 1190 (phosphine absorptions) cmꢀ1
,
due to 7. In addition, an absorption was also present
at 2040 cmꢀ1 due to excess Ni(CO)4. The solution was
immediately evaporated in vacuo, thus eliminating the
excess Ni(CO)4 with the solvent. Redissolution of part
of the obtained pale yellow solid in CH2Cl2 gave a solu-
tion showing in the IR spectrum the same signals men-
tioned above, with the exception of the band at 2040
cmꢀ1. Anal. Found: C, 50.09, H, 3.41. Calc. For
C21H15O3NiPS3: C, 50.33; H, 3.02. The compound is
very air sensitive even in the solid state and slowly dark-
ened during several weeks even if kept in a dinitrogen
atmosphere. If the reaction solution was not evaporated
after a few minutes, but stirred at room temperature
overnight, the solution darkened and a small amount
of a brown solid deposited on the walls of the tube.
The IR spectrum of the solution showed, apart from
the absorption at 2040 cmꢀ1 due to excess Ni(CO)4,
three absorptions in the carbonyl region at 2066 (m),
1993 (s), and 1942 (s), the first two of which virtually
coincident with the ones of 7. However, all three absorp-
tions were also present in the IR spectrum (in nujol) of
the brown residue [2067 (m), 2000 (s), 1943 (s)], indicat-
ing that all of them are due to the decomposition
product.
To a 100 ml Schlenk flask was added magnesium
(450 mg, 17.49 mmol, previously washed with diethyl
ether), and THF (7 ml). A solution of 3 (2.580 mg,
9.44 mmol) in THF (13 ml) was placed in a dropping
funnel and about 1/3 of the solution was quickly added
to the flask, followed by a few crystals of I2. The rest
of the solution was added during 2 h, after which the
flask was stirred at RT for an additional hour and then
cooled to 0 ꢁC. At this temperature, a solution of PCl3
(0.28 ml, 3.15 mmol) in THF (15 ml) was slowly added
by the same dropping funnel over 2 h. After the addi-
tion was over, 0.2 M HCl (10 ml) was added at once
and the biphasic solution stirred at RT overnight. Solid
NaCl (about 1 g) was added in order to improve sep-
aration between the two phases and the organic phase
was separated. The aqueous phase was extracted with
more THF (2 · 7 ml) and the combined organic phases
washed with water (15 ml) and dried with Na2SO4. The
oil after evaporation of the solvent was separated by
flash chromatography on silica (CH2Cl2 as eluent) to
give pure 4 (251 mg, 13% yield) as a pale yellow oil.
Anal. Found: C, 65.01; H, 6.60. Calc. for
C33H39O3PS3: C, 64.89; H, 6.44%.
4.9. Synthesis of Rh(acac)(CO)(P(4-C6H4SMe)3) (8)
H6
H7
P(C6H4SC5H9O)2
5'
H1
To a Schlenk tube were added Rh(acac)(CO)2 (18.0
mg, 0.070 mmol), P(4-C6H4SMe)3 (30.9 mg, 0.077
mmol), and toluene (10 ml). The solution was stirred
at RT for 2 days, after which an IR spectrum showed
complete disappearance of the starting complex. The
solution was concentrated in vacuo to about half vol-
H
O
S
H2
5H
4' H
H8
4 H
H
2'
H9
H3
H
3'

F. Ragaini et al. / Journal of Organometallic Chemistry 689 (2004) 3621–3630
3629
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