Electrospray mass spectrometric investigation of the reactivity of the sulfide centers in [Pt2(μ-S)2(PPh3) 4] towards organic dihalides and the catalytic potential of this complex in the syntheses of organosulfu

Article abstract of DOI:10.1016/S1381-1169(03)00308-X

Activity of [Pt₂(μ-S)₂(PPh₃) ₄] (1) towards 1,4-dichlorobutane, 1,2-dichloroethane, α-α′-dichloro-o-xylene and α-α′ -dichloro-m-xylene and the catalytic role of 1 in the syntheses of organosulfur materials was s

Full text of DOI:10.1016/S1381-1169(03)00308-X

Journal of Molecular Catalysis A: Chemical 204–205 (2003) 267–277
Electrospray mass spectrometric investigation of the reactivity of
the sulfide centers in [Pt2(␮-S)2(PPh3)4] towards organic dihalides
and the catalytic potential of this complex in the
syntheses of organosulfur materials
Siew Huay Chong, Antonius Tjindrawan, T.S. Andy Hor∗
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 23 October 2002; received in revised form 27 January 2003; accepted 3 February 2003
Dedicated to Professor Renato Ugo on the occasion of his 65th birthday
Abstract
Activity of [Pt2(␮-S)2(PPh3)4] (1) towards 1,4-dichlorobutane, 1,2-dichloroethane, ␣-␣ꢀ-dichloro-o-xylene and ␣-␣ꢀ-
dichloro-m-xylene and the catalytic role of 1 in the syntheses of organosulfur materials was studied by in situ electrospray mass
spectroscopy (ESMS). An intermediate, [Pt2(␮-SC2H4S)(PPh3)4][PF6]2 (6) was isolated and characterized by single-crystal
X-ray crystallographic diffractometry. In all reactions studied, the {Pt2S2} core is preserved, except for those involving excess
NaBH4. Treatment of 6 with excess NaBH4 leads to the formation of mononuclear complex [Pt(␩-S2C4H8)(PPh3)].
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Electrospray mass spectrometric; Sulfide; Organic dihalides; Catalytic; Organosulfur
1. Introduction
plying 1 in a metal-mediated process for the synthesis
of organosulfur materials (Scheme 1).
The sulfide centers in 1 can be functionalized with
organic dihalides to give 2. We have recently reported
that the selenium analogue of 1, viz. [Pt2(␮-Se)2-
(PPh3)4] reacts with excess 1,4-dibromobutane to give
1,6-diselenacyclodecane 3 as a white powder, albeit in
poor yields [7]. It would be valuable to develop a sim-
ilar process for 1 since many organosulfur compounds
are pharmacologically active. The catalytic value of 1
could be realized if we can recognize a mechanism to
return 2 to 1 by releasing a useful organosulfur sub-
strate and the platinum moiety. Details of the forma-
tion of 2 from 1, and the intermediates and forces in-
volved in regenerating 1 from 2 are key questions that
need to be addressed. In this paper, we use electrospray
Ugo et al. [1] and Chatt and Mingos [2] indepen-
dently developed [Pt2(␮-S)2(PR3)4], which has turned
out to be one of the best known building blocks of
homo-, hetero- and intermetallic sulfide aggregates to-
day [3]. The PPh3 derivative, [Pt2(␮-S)2(PPh3)4] (1)
has been much better studied, despite its ready decom-
position in chlorinated solvents [4]. Under controlled
conditions, such disintegration can be harnessed and
the sulfide becomes a key in the functionalization pro-
cess [5,6]. We intend to develop this concept by ap-
∗
Corresponding author. Tel.: +65-6874-2658;
fax: +65-6779-1691.
E-mail address: andyhor@nus.edu.sg (T.S.A. Hor).
1381-1169/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00308-X

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Scheme 1. A metal-mediated catalytic-like and stoichiometric synthesis of organosulfur materials.
mass spectrometry (ESMS) to study the feasibility of
these conversions and to determine the active inter-
mediates involved. The development of ESMS as a
rapid and convenient tool to probe the chemistry of 1
with metal substrates has been discussed [5] and many
successful ESMS-aided syntheses have been realized
[8,9].
tope distributed patterns. The m/z values given are
for the most intense peak in the envelopment in each
case. The 31P NMR spectra were recorded at 25 ◦C
on a Bruker ACF 300 spectrometer at 121.50 MHz
with 85% H3PO4 as external reference.
2.2. Materials
All dihalide substrates were obtained directly from
Aldrich. Solvents used were of analytical grade and
used as received. All reactions were performed under
pure dry nitrogen unless otherwise stated. Complex 1
was synthesized via metathesis of cis-[PtCl2(PPh3)2]
with Na2S·9H2O in benzene [2,10]. ESMS (100%
MeOH): m/z = 1503 [M + H]+.
2. Experimental
2.1. Instrumentation
Solutions obtained from the reaction flask for
ESMS analysis were diluted 10 times in 100% MeOH
solution. Any suspension was filtered prior to dilu-
tion. Electrospray mass spectra were obtained with
a Finnigan/MAT LCQ mass spectrometer coupled
with TSP4000 HPLC system and the crystal 310 CE
system. The mobile phase is 100% MeOH pumped
at a flowrate of 0.4 ml/min. Samples were injected
via a Rheodyne valve fitted with a 5 ␮l sample
loop. The capillary temperature was 200 ◦C and the
capillary voltage was 11 V. The capillary potential
tip was 4500 V, with nitrogen used both as a sheath
(4134 Torr) and an auxiliary gas (1034 Torr). Peaks
were assigned from the m/z values and from the iso-
2.2.1. Reaction of 1 with 1,4-dichlorobutane
Compound 1 (40.0 mg, 0.0266 mmol) was intro-
duced into degassed methanol (20 ml), followed by
1,4-dichlorobutane (0.40 ml, 465.4 mg, 3.593 mmol,
100-fold excess). The mixture changed to a yellow
solution after 24 h stirring under N2. A sample of
the clear solution was analyzed by ESMS. Excess
NH4PF6 (20.0 mg, 0.1227 mmol) was added and
the solution was stirred for 1 h. Deionized water
(50 ml) was added to give precipitation of a yellow
oil, which was extracted with C6H6 followed by

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269
concentration in vacuo to give light-yellow powder
(Yield: 14.77 mg).
stirring for 3 h, a sample of the solution was analyzed
by ESMS.
2.2.2. Reaction of 1 with 1,2-dichloroethane
2.3. Crystal structure determinations
By using a similar procedure as in Section 2.2.1,
excess 1,2-dichloroethane (0.30 ml, 375.9 mg, 3.799
mmol, 100-fold excess) and 40.0 mg (0.0266 mmol)
of compound 1 gave a yellow suspension. A sample
of it was filtered and analyzed by ESMS.
The data collection was performed on a Bruker
AXS APEX diffractometer, equipped with a CCD
area-detector using Mo K␣ radiation (λ = 0.71073 Å).
The software SMART [11] was used for collecting
frames of data, indexing reflections, and the deter-
mination of lattice parameters, SAINT [11] for inte-
gration of intensity of reflections and scaling, SAD-
ABS [12] for empirical absorption correction, and
SHELXTL [13] for space group and structure deter-
mination, refinements, graphics, and structure report-
ing. Hydrogen atoms were not located. The structure
was refined by full-matrix least squares on F2 with
anisotropic thermal parameters for non-hydrogen
atoms. A summary of parameters for the data collec-
tion−s and refinements is given in Table 1. One of the
PF6 ions and the solvate (CHCl3) are disordered.
The quality of the data does not merit a detailed
2.2.3. Synthesis of [Pt2(µ-S2C2H4)(PPh3)4][PF6]2
(6)
Compound 1 (40.0 mg, 0.0266 mmol) was intro-
duced into degassed methanol (10 ml), followed by
addition of 1,2-dichloroethane (0.30 ml, 100-fold
excess). The mixture readily changed from a light
orange to yellow suspension. The suspension was
then refluxed overnight under N2 at 60 ◦C to give a
clear light-yellow solution. Excess NH4PF6 (30.0 mg,
0.184 mmol) was added, readily turning the solution
into a white suspension. After stirring for another
2 h, 50 ml of deionized water was added to induce
the precipitation. The solid was obtained using vac-
uum suction filtration and washed with 200 ml of
deionized water and 100 ml of diethyl ether to yield
a light-yellow powder (Yield: 43.14 mg). The solid
was dissolved in chloroform and layered with hexane,
and white crystals suitable for X-ray analysis were
obtained.
Table 1
Selected crystallographic data for [Pt2(␮-S2C2H4)(PPh3)4]2+
[PF6]2·6CHCl3·H2O
-
6·6CHCl3·H2O
Formula
C80H72Cl18F12P6Pt2S2O
fw
2555.92
0.20 × 0.18 × 0.04
Monoclinic
P2(1)/c
18.544(3)
18.300(3)
28.376(4)
90
99.964(4)
90
9484(2)
4
Crystal size (mm3)
Crystal system
Space group
a (Å)
2.2.4. Reaction of 1 with α-αꢀ-dichloro-o-xylene
By using a similar procedure as in Section 2.2.1, ex-
cess ␣-␣ꢀ-dichloro-o-xylene (468.6 mg, 2.676 mmol,
100-fold excess) and 40.0 mg (0.0266 mmol) of com-
pound 1 gave a yellow solution after 10 min. After
stirring for 3 h, a sample was analyzed by ESMS. Ex-
cess NH4PF6 (20.0 mg, 0.122 mmol) was added and
the solution was stirred for 1 h. Deionized water of
50 ml was added to induce the precipitation. The white
precipitate (26.76 mg) obtained using vacuum suction
filtration was washed with 100 ml of deionized water
and 30 ml of diethyl ether.
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3)
Z
µ (mm−1
T (K)
)
3.667
223(2)
1.65–22.50
θ range (◦)
Index ranges
−19 < h < 19
−19 < k < 19
−26 < l < 30
2.2.5. Reaction of 1 with α-αꢀ-dichloro-m-xylene
By using a similar procedure as in Section 2.2.1, ex-
cess ␣-␣ꢀ-dichloro-m-xylene (465.8 mg, 2.661 mmol,
100-fold excess) and 40.0 mg (0.0266 mmol) of com-
pound 1 gave a yellow solution after 10 min. After
No. of reflections collected
No. of unique reflections
R1 (I > 2σ(I))
42490
12380
0.1094
0.2939
wR2 (all data)

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discussion of the bond data. However, there is little
doubt on the identity of the molecular structure.
turns to a clear solution. The observed ion isotope
distribution is compared with simulated isotope pat-
terns of the identified species. The presence of peaks
due to 1 would be interpreted as a non- or incom-
plete reaction. All systems in general give mono-
and di-cationic sulfur-derivatized complexes due to
nucleophilic attack at one or both of the two C–Cl
sites. The dinuclear {Pt2S2} core is preserved in all
cases. Addition of a strong reducing agent, NaBH4 to
the di-cationic sulfur-derivatized complexes however,
cleaves the Pt–S bonds and leads to disintegration of
the core.
3. Results and discussion
3.1. General conditions in the use of ESMS in
screening reactions between [Pt2(µ-S)2(PPh3)4] 1
and organic dihalides
[Pt2(␮-Se)2(PPh3)4] reacts with organic dihalides
such as 1,2-dichloroethane, 1,4-dichlorobutane and
␣-␣ꢀ-dichloro-o-xylene to yield stable di-cationic
selenolato products [8,9]. These bifunctional sub-
strates are expected to activate the sulfide centers of
1 en route to organosulfur precursors. Positive-mode
ESMS was employed to monitor the progress of re-
actions. To maintain consistency, the same detection
conditions are applied to all systems under study.
Analysis takes place as soon as the reaction mixture
3.1.1. Reactivity of 1 with aliphatic dihalides
Reaction between excess 1,4-dichlorobutane and
1 in MeOH at RT gives rise to a clear yellow so-
lution, which indicates facile formation of ionic
products. The ES mass spectrum of this solu-
tion, after 24-h, is shown in Fig. 1. It consists
of peaks at m/z = 1594 (major) and 780 (minor)
Fig. 1. Positive ion electrospray mass spectrum of a solution of [Pt2(␮-S)2(PPh3)4] (1) with 1,4-dichlorobutane in MeOH after 24 h of
reaction.

S.H. Chong et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 267–277
271
corresponding to [Pt2(␮-S)(␮-SC4H8Cl)(PPh3)4]+
(4) and [Pt2(␮-S2C4H8)(PPh3)4]2+ (5), respectively.
This demonstrates a clear step-wise alkylation and
that both sulfur sites are susceptible to attack. Frag-
mentation of PPh3 ligands from 4 is minimal.
(1JPt–P = 2907 Hz)) and 4 (multiplets at 2989 Hz
1
(1JPt–P(1) = 3218 Hz and JPt–P(2) = 2608 Hz).
The phosphines trans to the electron-withdrawing
␮-S-C4H8Cl ligand should give rise to the higher
1JPt–P(1) value (3218 Hz).
Using a similar dihalide but with a shorter alkyl
chain length, viz. 1,2-dichloroethane under similar
conditions gave a yellow suspension, whose mass
spectrum (Fig. 3) shows strong signals pertaining to
the starting material 1 (m/z = 1503 for [1 + H]+;
1241 for [1 + H − (PPh3)]+; and 979 for [1 + H
− (PPh3)2]+), which are absent in Figs. 1 and 2.
The di-cationic species [Pt2(␮-S2C2H4)(PPh3)4]2+
(6) can be detected as a weaker signal at m/z = 766.
After 24 h, the ES mass spectrum of the reaction
mixture shows the signal at m/z = 766 (6) is more
intense but the starting materials are still prominent.
After 72 h, the di-cation 6 became the dominant
species. This prolonged reaction time also led to
a new peak at m/z = 1304 (Fig. 4), attributed to
A yellow oil could be obtained from the solution
upon metathesis with NH4PF6. In MeOH, this oil
gave a spectrum that is predominantly 5 (Fig. 2). Its
31P NMR spectrum is consistent with the formation
of 5 (a singlet at 2612 Hz with the associated satellite
Fig. 2. Positive ion electrospray mass spectrum of the yellow oil obtained after solution of 1 with 1,4-dichlorobutane in MeOH had been
metathesized.

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Fig. 3. Positive ion electrospray mass spectrum of a mixture of 1 with 1,2-dichloroethane in MeOH. Time taken for reaction: 24 h (RT).
[Pt2(␮-S2C2H4)(PPh3)3Cl]+ (7), whose formation
suggests that phosphine dissociation and entry of
chloride into the molecular core would represent
a possible side reaction in the proposed catalytic
methodology. At higher temperatures, the reaction
proceeds faster, as supported by the absence of 1
in the spectrum of the clear yellow solution ob-
tained after heating the reaction mixture under reflux
for 24 h. A light-yellow powder was obtained after
metathesis with NH4PF6. 31P NMR analysis, with a
3.1.2. Reactivity of 1 with xylene dihalides
Reactions using aromatic and/or bulkier dihalides
singlet at 1883 Hz (1JPt–P = 3098 Hz), suggested it
to be [Pt2(␮-S2C2H4)(PPh3)4]2+ (6). Single-crystal
crystallographic diffraction analysis confirmed a
diplatinum core bridged by an ethanedithiolato
(–SCH2CH2S–) group (Fig. 5; Tables 1 and 2).
This marks the first step in the catalytic-like cycle
(Scheme 1) in which sulfide is functionalized to
give a dithiolato moiety. The stability and activity
of this moiety is a subject of our on-going investi-
gations.
such as ␣-␣ꢀ-dichloro-o-xylene and ␣-␣ꢀ-dichloro-m-
xylene proceed much more rapidly; they tend to give
clear solutions at RT within 30 min. After treatment
of ␣-␣ꢀ-dichloro-o-xylene with 1 for 3 h at RT, the
resulting solution in the ES mass spectrum gave an
intense peak at m/z = 1379 (Fig. 6), inferring the
formation of [Pt2(␮-S2CH2C6H4CH2)(PPh3)3Cl]+
(8), which is analogous to 7. The expected di-cation
[Pt2(␮-S2CH2C6H4CH2)(PPh3)4]2+ (9) (m/z = 804)

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273
Fig. 4. Positive ion electrospray mass spectrum of a mixture of 1 with 1,2-dichloroethane in MeOH after 72 h reaction at RT.
is also prominent, and this species can be isolated as
a white precipitate by metathesis with NH4PF6. How-
ever, unlike 6, there are two sets of phosphines (from
31P NMR analysis), which arise from unsymmetrical
disposition of the xylene moiety. Similar behavior has
been discussed in the selenium analogue [9].
dichloro-m-xylene and, as expected, gives predom-
inantly the monocation [Pt2(␮-S)(␮-SCH2C6H4-
CH2Cl)(PPh3)4]+ (10) (m/z = 1642), and some frag-
mentations (Fig. 7). Such substrates would be more
suitable for organo(mono)sulfur syntheses whereas
the dithiolato complexes could be more suitable for
(di)sulfur macrocycles. These latter compounds are
the target of our current study.
A substrate that is not suitable to serve as an
overhead bridge between the sulfur centers, is ␣-␣ꢀ-

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Fig. 5. ORTEP plot of the cation of the molecular structure of [Pt2(␮-S2C2H4)(PPh3)4][PF6]2·6CHCl3·H2O with 50% probability thermal
ellipsoid (anions and solvate are removed for clarity).
Fig. 6. Positive ion electrospray mass spectrum of a mixture of 1 with ␣-␣ꢀ-dichloro-o-xylene in MeOH upon 3 h reaction.

S.H. Chong et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 267–277
275
Fig. 7. Positive ion electrospray mass spectrum of a mixture of 1 with ␣-␣ꢀ-dichloro-m-xylene in MeOH upon 3 h reaction.
Scheme 2. Disintegration of the {Pt2S2} core in [Pt2(␮-S2C2H4)(PPh3)4]2+ (6) under excess NaBH4.

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Table 2
3.1.3. Organosulfur materials
Selected bond lengths (Å) and angles (◦) for [Pt2(␮-S2C2H4)-
(PPh3)4]2+[PF6]2·6CHCl3·H2O
We have yet to detect free organosulfur macrocy-
cles in the reaction mixtures. As this is an objective
of our approach, we should investigate the strength
of the Pt–S bonds in di-cationic 2. In the presence
of excess NaBH4, [Pt2(␮-S2C4H8)(PPh3)4]2+ (5)
(RT, 4 h) disintegrates to [Pt(PPh3)(PPh2C6H4)]+
(11) (m/z = 718) (major) and [4–(PPh3)–S]+ (m/z
= 1297) (minor) (Scheme 2). However, when
[Pt2(␮-S2C2H4)(PPh3)4]2+ (6) is treated with excess
NaBH4 (RT, 24 h), Pt–S cleavage and formation of
[Pt(␩-S2C4H8)(PPh3)] (12) (m/z = 812) is evident
(Fig. 8). Its formation is encouraging as it could
provide a pathway for the [Pt(PPh3)2] moiety to be
re-cycled (Scheme 1) and in doing so, generate a new
dithiolato complex that can be used for the synthesis
of organo(di)sulfur substrates.
6·6CHCl3·H2O
Bond lengths
Pt(1)–P(1)
Pt(1)–P(2)
Pt(1)–S(1)
Pt(1)–S(2)
Pt(2)–S(1)
2.294(6)
2.274(6)
2.373(6)
2.352(6)
2.359(6)
2.375(6)
1.83(2)
Pt(2)–S(2)
S(1)–C(1)
S(2)–C(2)
1.88(2)
Bond angles
Pt(1)–S(2)–Pt(2)
Pt(2)–S(1)–Pt(1)
S(1)–Pt(2)–S(2)
P(2)–Pt(1)–P(1)
P(1)–Pt(1)–S(1)
P(1)–Pt(1)–S(2)
P(2)–Pt(1)–S(1)
P(2)–Pt(1)–S(2)
C(1)–S(1)–Pt(1)
C(1)–S(1)–Pt(2)
87.4(2)
87.3(2)
76.9(2)
99.1(2)
89.5(2)
166.6(2)
169.8(2)
94.1(2)
94.5(8)
96.7(9)
Fig. 8. Positive ion electrospray mass spectrum of a mixture of [Pt2(␮-S2C2H4)(PPh3)4]2+ (6) with NaBH4 in MeOH after 24 h reactions.

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277
4. Conclusion
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The catalytic cycle proposed earlier is decep-
tively simple. Recent results suggested that for
[Pt2(␮-S)2(PPh3)4] (1) to have a significant catalytic
impact, we need to promote its subsequent alkylation,
and balance it with controlled Pt–S cleavage. We are
encouraged by the variation in terms of single alky-
lation (versus double alkylation), monocation (versus
di-cation) formation, and the preservation (versus dis-
integration) of the {Pt2S2} core. The use of ESMS
allowed us to observe these parallel and sometimes
conflicting processes in solution. With an assortment
of organic dihalides at hand, we are now able to study
the subtle effects that the organic overhead can impart
to these processes. In doing so, we are one step closer
to exploiting this metal-mediated methodology in the
design and synthesis of organosulfur substrates.
Acknowledgements
[12] G.M. Sheldrick, SADABS,
a software for empirical
adsorption correction, University of Gottingen, 1993.
[13] G.M. Sheldrick, SHELXTL, Version 5.03, Siemens Energy &
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The authors are indebted to L.L. Koh and G.K.
Tan for their help in solving the crystal structure. We
acknowledged Agency for Science, Technology and
Research (A∗STAR) for a grant (012-101-0035). This
paper is dedicated to Professor Renato Ugo for his
pioneering work on [Pt2(␮-S)2(PPh3)4].