Unusual carbon-sulfur bond cleavage in the reaction of a new type of bulky hexathioether with a zerovalent palladium complex

Article abstract of DOI:10.1039/b513339d

The reaction of a bulky hexathioether, TbtS(o-Phen)S(o-Phen)SS(o-Phen)S(o- Phen)STbt (o-Phen = o-phenylene, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl] phenyl) (1), with 3 molar amounts of Pd(PPh₃)₄ afforded trinuclear palladium comp

Full text of DOI:10.1039/b513339d

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Unusual carbon–sulfur bond cleavage in the reaction of a new type of
bulky hexathioether with a zerovalent palladium complex{
Daisuke Shimizu, Nobuhiro Takeda and Norihiro Tokitoh*
Received (in Cambridge, UK) 21st September 2005, Accepted 21st October 2005
First published as an Advance Article on the web 21st November 2005
DOI: 10.1039/b513339d
The reaction of
a bulky hexathioether, TbtS(o-Phen)-
S(o-Phen)SS(o-Phen)S(o-Phen)STbt (o-Phen = o-phenylene,
Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) (1), with 3
molar amounts of Pd(PPh3)4 afforded trinuclear palladium
complex 4 bridged by two benzenedithiolato ligands via a three-
step palladium insertion reaction into one sulfur–sulfur and two
carbon–sulfur bonds of 1.
The transition-metal-mediated activation of a C–S bond has been
paid great attention from the standpoints of synthetic chemistry,
petrochemical hydrodesulfurization, and bioinorganic chemistry.1
As far as C(sp2)–S activation is concerned, a number of examples
of thiophene derivatives have been studied and well established.2
In contrast, there are relatively few reports on aryl C(sp2)–S bond
activation.3 On the other hand, as an extension of our study on the
sterically crowded organoelement compounds, we have recently
synthesized a new bulky hexathioether ligand possessing a
disulfide moiety, TbtS(o-Phen)S(o-Phen)SS(o-Phen)S(o-Phen)-
STbt (o-Phen = o-phenylene, Tbt = 2,4,6-tris[bis(trimethylsilyl)-
methyl]phenyl) (1).4
Scheme 1
the thiol intermediate 3. Oxidative coupling reaction of 3 using
iodine was performed to give the expected hexathioether 1 as
colorless crystals in 93% yield. The thiol intermediate 3 was
similarly oxidized with air instead of iodine to give 1 in a
comparable yield. Hexathioether 1 was characterized by the NMR
spectra, MS, and elemental analysis, and its molecular structure in
the crystalline state was determined by X-ray structural analysis.{
The ORTEP drawing of 1 is shown in Fig. 1. Hexathioether 1 is
made up of the interlinked sulfide parts and the central disulfide
part. All the C–S bond lengths of the diarylsulfide moieties of 1
˚
[1.782(4) A for C(1)–S(1), 1.772(4) A for C(28)–S(1), 1.787(4) A for
˚
˚
˚
˚
C(33)–S(2), 1.781(4) A for C(34)–S(2), 1.779(4) A for C(45)–S(5),
˚
˚
˚
1.773(4) A for C(46)–S(5), 1.774(4) A for C(51)–S(6), 1.784(4) A
for C(52)–S(6)] are within the range of C–S distances reported for
diarylsulfides.5 The lengths of the C–S and S–S bonds of the
This hexathioether ligand 1 has two different reactive sites
toward transition metals, namely, the thioether moieties and the
disulfide moiety. While this ligand is considered to act simply as a
Lewis base at its thioether sites, it may undergo cleavage of the S–S
bond and the formation of metal–sulfur bonds when reacted via
the disulfide site. Therefore, it will be a promising ligand for
application to the synthesis of novel transition metal complexes. In
this paper, we present the synthesis of 1 and its reactions with low-
valent palladium(0) metal complexes.
˚
diaryldisulfide moiety of 1 [1.794(4) A for C(39)–S(3), 1.792(4) A
˚
Hexathioether 1 was synthesized by the reactions shown in
Scheme 1. The coupling reaction of TbtSH with 1,2-diiodobenzene
using Cu2O in refluxing 2,4,6-trimethylpyridine afforded the
corresponding Tbt-substituted sulfide 2 in 52% yield. Further
coupling reaction of sulfide 2 with 1,2,-benzenedithiol under
conditions similar to the above coupling reaction using Cu2O gave
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto
611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp;
Fax: +81 774 38 3209; Tel: +81 774 38 3200
{ Electronic supplementary information (ESI) available: experimental
details. See DOI: 10.1039/b513339d
Fig. 1 ORTEP drawing of 1 (50% probability). The hydrogen atoms and
solvent molecules are omitted for clarity.
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Chem. Commun., 2006, 177–179 | 177

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Scheme 2
˚
for C(40)–S(4), and 2.0364(15) A for S(3)–S(4)] are also close to
˚
those observed in common diaryldisulfides [1.754–1.789 A for aryl
6
˚
C–S bonds and 2.023–2.099 A for S–S bonds].
With the new hexathioether ligand 1 (bearing extremely bulky
terminal substituents) in hand, we examined the complexation of 1
with [Pd(PPh3)4] (Scheme 2). To a benzene solution of 1 was added
3 molar amounts of [Pd(PPh3)4], and the resulting yellow solution
was stirred at room temperature for 62 h. The crude product was
purified by silica gel column chromatography to afford trinuclear
Pd complex 4 as yellow crystals in 58% yield.
The formation of tripalladium complex 4 was most likely
interpreted in terms of the three-step palladium insertion reaction
into the one S–S and the two C–S bonds of hexathioether 1. Since
the transition metal insertion reaction into the S–S bond is one of
the typical reactions of disulfides,7 the S–S bond cleavage of
hexathioether 1 is not so surprising. However, as mentioned above,
the transition-metal-mediated aryl C(sp2)–S bond cleavage is very
scarce and unprecedented by a palladium(0) metal complex, albeit
a number of sulfur-coordinated palladium complexes have been
synthesized so far.8 This is, to the best of our knowledge, the first
example of an oxidative addition of a palladium(0) metal complex
toward an aryl C(sp2)–S bond.
Fig. 2 ORTEP drawings of 4 (50% probability): (i) top view and (ii) view
from the side of a central PdS4 plane. The hydrogen atoms and solvent
molecules are omitted for clarity.
Pd(1)*) in the Pd3S4 core could be treated as pseudo-distorted
octahedral structures in the crystalline state.
…
˚
The Pd(1) Pd(2) distance [3.194 A] is longer than the sum of
12
the metallic radii (2.76 A for Pd), indicating no direct metal–
˚
Single crystals of 4 were obtained by recrystallization from
chloroform and benzene solution. The molecular structure of 4
was determined by X-ray crystallographic analysis,{ and the
ORTEP drawing of 4 is shown in Fig. 2. The three palladium
atoms are linked by two bridging o-benzenedithiolato ligands to
form the Pd3S4 core. Consequently, the palladium complex 4 is
classified as a new type of tetrakis(m-thiolato)tripalladium complex.
The central palladium atom is coordinated by four sulfur atoms of
the two o-benzenedithiolato ligands with a slightly distorted
square-planar geometry. The two bite angles for S(1)–Pd(2)–S(2)
and S(1)–Pd(2)–S(2)* are 89.95u and 89.62u, respectively. The Pd–S
metal interaction. It is noteworthy that the Pd(1)–Pd(2)–Pd(1)*
angle [122.6u] differs substantially from those in most of the known
tetrakis(m-thiolato)tripalladium complexes (173.2–180u). They have
been known to have linear tripalladium structures, in which the
palladium atoms are bridged by four sulfur atoms at nonbonding
Pd–Pd distances. The palladium complex 4 is the second example
of the tetrakis(m-thiolato)tripalladium complexes having a bent
tripalladium structure. [Pd3(m-SCy)4Cl2(PMe3)2]13 is the only other
example of a structurally characterized bent tripalladium complex
(132.2u). The dihedral angles between the coordination plane of
Pd(2) and those of Pd(1) and Pd(1)* [about 136u] are nearly equal
to those of [Pd3(m-SCy)4Cl2(PMe3)2] (130 and 131u). This bent
˚
˚
bond lengths [2.2873(10) A for Pd(2)–S(1) and 2.2798(10) A for
Pd(2)–S(2)] are slightly shorter than the values of the previously
reported tetrakis(m-thiolato)tripalladium complexes (2.318–
9
2.387 A). On the other hand, the two lateral palladium atoms
˚
of the Pd3S4 core are coordinated by two sulfur atoms, one
phosphorus atom, and one carbon atom in a more distorted
square-planar geometry than that of the central Pd atom. This
distortion is considered to be caused by the steric repulsion
between the bulky Tbt and PPh3 groups. Interestingly, as shown in
…
Fig. 3, the nonbonded distances for Pd(1) S(3) and
…
˚
Pd(1)* S(3)* [3.090 A] are shorter than the sum of the van der
Waals radii of palladium and sulfur atoms (3.43 A).10 In addition,
˚
…
the nonbonded distances for Pd(1) C(45) and Pd(1)* C(45)*
…
˚
[3.339 A] are approximately the same as the sum of the van der
˚
Waals radii of palladium and carbon atoms (3.33 A), suggesting
…
the existence of weak interactions in Pd(1) H(72) and
11
…
Pd(1)* H(72)*. So the two palladium atoms (Pd(1) and
Fig. 3 The partial structure of 4.
178 | Chem. Commun., 2006, 177–179
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3
V = 14471.3(5) A , Z = 4, Dcalc = 1.281 g cm23, R1(I > 2s(I)) = 0.0438,
wR2(all data) = 0.0936, T = 103(2) K, GOF = 1.058. The structures were
solved by a direct method (SIR-97)14 and refined by full-matrix least-
squares procedures on F2 for all reflections (SHELXL-97).15 CCDC 285411
and 285412. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513339d
˚
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Fig. 4 Variable temperature-31P{1H}NMR spectra of 4 in benzene-d6:
(a) 25 uC, (b) 30 uC, (c) 40 uC, (d) 50 uC, (e) 60 uC, (f) 70 uC.
structure of 4 may result from steric interactions of bulky
substituents, such as Tbt and PPh3 groups, as well as in the case
of [Pd3(m-SCy)4Cl2(PMe3)2].
3 N. F. Ho, T. C. W. Mak and T.-Y. Luh, J. Organomet. Chem., 1986,
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1
The H, 13C, and 31P NMR spectra of complex 4 measured at
ambient temperature were not consistent with the structure
determined by X-ray crystallographic analysis. For example, the
1H NMR spectrum showed four nonequivalent methine protons
assignable to the ortho benzyl protons of the two Tbt groups and
considerably broad signals for PPh3 groups. The 31P{1H} NMR
spectrum of 4 in benzene-d6 exhibited two signals (d = 28.5 and
28.7) for the PPh3 groups at 25 uC (Fig. 4). The 31P{1H} NMR
spectrum at 70 uC exhibited one sharp signal (d = 28.5) and the 1H
and 13C NMR spectra of the same temperature could be
reasonably attributed to the structure observed in the crystalline
state. In addition, after heating the samples, there was no change in
the NMR spectra at 25 uC. These results may stem from the
existence of some isomers at 25 uC due to its overcrowded
structure bearing bulky Tbt and PPh3 groups, although we have
no definitive interpretation for the behavior of the NMR spectra.
In summary, we have synthesized a new, bulky hexathioether 1
containing a disulfide moiety and found unusual C–S bond
cleavage by the reaction of 1 with 3 molar amounts of Pd(PPh3)4
to give an interesting trinuclear palladium complex 4 with a novel
structure. The studies on the formation mechanism and the
intrinsic properties of 4 are currently in progress.
4 A part of this work has been preliminarily reported: N. Tokitoh,
D. Shimizu, N. Takeda and T. Sasamori, Phosphorus, Sulfur Silicon,
2005, 180, 1241–1245.
5 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
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8 Recently phenyl C(sp2)–S bond cleavages using palladium nanoparticles
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126, 7386–7392.
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This work was partially supported by Grants-in-Aid for
Scientific Research [Nos. 12CE2005, 17GS0207, 14078213, and
15750031] and the 21 COE Program on Kyoto University Alliance
for Chemistry from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We are grateful to Dr Takahiro
Sasamori, Institute for Chemical Research, Kyoto University, for
valuable discussions.
10 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
…
…
˚
11 The nonbonded lengths of Pd(1) H(72) and Pd(1)* H(72)* [2.755 A]
are shorter than the sum of the van der Waals radii of palladium and
˚
hydrogen atoms (2.90 A). All hydrogen atoms were treated as riding,
˚
with C–H distances of 0.95 A.
Notes and references
12 W. H. Zachariasen, J. Inorg. Nucl. Chem., 1973, 35, 3487–3497.
13 E. M. Padilla and C. M. Jensen, Polyhedron, 1991, 10, 89–93.
14 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano,
C. Giacovazzo, A. Guagliardi, A. G. G. Molitemi, G. Polidori and
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{ Crystal data for 1: C81.50H142ClS6Si12, M = 1686.84, triclinic, space group
¯
˚
P1 (no. 2), a = 10.920(2), b = 20.424(5), c = 23.068(5) A, a = 71.290(7), b =
,
R1(I > 2s(I)) = 0.0647, wR2(all data) = 0.1661, T = 103(2) K, GOF = 1.081;
3
83.335(10), c = 83.675(11)u, V = 4825.5(18) A , Z = 2, Dcalc = 1.161 g cm23
˚
4: C134H184Cl2.40P2Pd3S6Si12, M = 2790.47, monoclinic, space group C2/c
˚
15 G. M. Sheldrick, SHELX-97, Program for the Refinement of Crystal
Structures, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(no. 15), a = 31.9316(5), b = 24.7082(6), c = 24.0157(5) A, b = 130.2043(8)u,
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 177–179 | 179