Reinvestigation of the Pd-catalyzed bis(silylation) of alkynes with 1,1,2,2-tetramethyl-1,2-bis(phenylthiomethyl)disilane: Unexpected formation of the eight-membered siloxane-chelate complex cis-[PdCl2{(PhSCH 2SiMe2)2O}]

Article abstract of DOI:10.1016/j.jorganchem.2012.11.022

The bis(silylated) alkenes Z-(PhSCH₂)Me₂SiC(H)C(Fc) SiMe₂(CH₂SPh) (2) and Z-(PhSCH₂)Me ₂SiC(H)C(bipheny)SiMe₂(CH₂SPh) (3) have been prepared by Pd-catalyzed double silylation of ethynylferrocene and 4-ethynyl-1,1′-biphenyl in the presence of 1,1,2,2-tetramethyl-1,2- bis(phenylthiomethyl)disilane (1). A reinvestigation on the interaction of 1 with [PdCl₂(PhCN)₂] in technical-grade CH ₂Cl₂ as solvent revealed competition between reduction to elemental palladium (due to oxidative addition of the Si-Si bond across Pd(II) and subsequent reductive elimination) and formation of an unusual eight-membered chelate complex cis-[PdCl₂{(PhSCH₂SiMe₂) ₂O}] (4), which is fluxional in solution. Based on GC-MS analyses revealing both formation of chlorosilane ClSiMe₂CH₂SPh and the disiloxane (PhSCH₂SiMe₂)₂O, residual water contained in CH₂Cl₂ is supposed to be the source for the oxygen atom of the siloxane function. The mechanism of the formation of 4 is discussed. The molecular structures of 1 and 4 were determined by single-crystal X-ray diffraction analyses. The energies of several conformational isomers of 4 were computed by means of DFT calculations, the coexistence of a species featuring a weak dative Pd-O interaction due to a hemi-labile coordination of ligand 1 was evidenced.

Full text of DOI:10.1016/j.jorganchem.2012.11.022

Journal of Organometallic Chemistry 724 (2013) 262e270
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Reinvestigation of the Pd-catalyzed bis(silylation) of alkynes with 1,1,2,
2-tetramethyl-1,2-bis(phenylthiomethyl)disilane: Unexpected formation of the
eight-membered siloxaneechelate complex cis-[PdCl2{(PhSCH2SiMe2)2O}]
,
,
b
a
Harmel W. Peindy N’dongo a b, Sébastien Clément b , Sébastien Richeter , Fabrice Guyon ,
*
Michael Knorr a , Pierre le Gendre , Jonathan O. Bauer , Carsten Strohmann
,
c
d
d,
**
*
a Institut UTINAM UMR CNRS 6213, Université de Franche-Comté, Faculté des Sciences et des Techniques, 16 Route de Gray, 25030 Besançon, France
b Institut Charles Gerhardt, UMR CNRS 5253, Equipe Chimie Moléculaire et Organisation du Solide, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France
c Institut de Chimie Moléculaire, UMR CNRS 6302, Université de Bourgogne, 21078 Dijon, France
d Institut für Anorganische Chemie der Technischen Universität Dortmund, Otto-Hahn-Strasse 6, 44227 Dortmund, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The bis(silylated) alkenes Z-(PhSCH2)Me2SiC(H)]C(Fc)SiMe2(CH2SPh) (2) and Z-(PhSCH2)Me2SiC(H)]
C(bipheny)SiMe2(CH2SPh) (3) have been prepared by Pd-catalyzed double silylation of ethynylferrocene
and 4-ethynyl-1,10-biphenyl in the presence of 1,1,2,2-tetramethyl-1,2-bis(phenylthiomethyl)disilane (1).
A reinvestigation on the interaction of 1 with [PdCl2(PhCN)2] in technical-grade CH2Cl2 as solvent
revealed competition between reduction to elemental palladium (due to oxidative addition of the SieSi
bond across Pd(II) and subsequent reductive elimination) and formation of an unusual eight-membered
chelate complex cis-[PdCl2{(PhSCH2SiMe2)2O}] (4), which is fluxional in solution. Based on GCeMS
analyses revealing both formation of chlorosilane ClSiMe2CH2SPh and the disiloxane (PhSCH2SiMe2)2O,
residual water contained in CH2Cl2 is supposed to be the source for the oxygen atom of the siloxane
function. The mechanism of the formation of 4 is discussed. The molecular structures of 1 and 4 were
determined by single-crystal X-ray diffraction analyses. The energies of several conformational isomers
of 4 were computed by means of DFT calculations, the coexistence of a species featuring a weak dative Pd
eO interaction due to a hemi-labile coordination of ligand 1 was evidenced.
Received 26 July 2012
Received in revised form
9 November 2012
Accepted 14 November 2012
Keywords:
Palladium
Thioether complexes
Siloxane
Disilane
Ethynylferrocene
DFT
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
hexamethyldisilane can be oxidatively added across [Pt(PEt3)3] to
provide cis-[Pt(Me3Si)2(PEt3)2] [6]. The Pd-catalyzed reaction of
Disilanes, notably functionalized disilanes, are more and
more used as versatile reagents in organometallic coordination
chemistry and catalytic transformations. For example, HR2SieSiR2H
(R ¼ Me, Ph) have been used in oxidative addition reactions across
[(MeCp)Mn(CO)2(THF)], [CpRe(CO)2(THF)] and [Fe(CO)4PPh3] for
the synthesis of hydrido disilanyl complexes [1]. Disilanyl
complexes [CpM(CO)3Si2Me5] (M ¼ Cr, Mo, W) have also been
alternatively prepared by reaction of [CpM(CO)3]Na with Me5Si2Cl
[2e4]. In the course of these stoichiometric reactions, a cleavage of
the SieSi bond of disilanes may also occur. For example, treatment
of HMe2SieSiMe2H with [Pt(PEt3)3] has led to the bis(silyl) complex
cis-[Pt(HMe2Si)2(PEt3)2] [5]. Even the unactivated disilane
aroyl chlorides with hexamethyldisilane using [PdCl2(PhCN)2] as
catalyst represents a high-yield synthetic route leading to aroylsi-
lanes [7].
However, the most wide-spread application is the transition
metal mediated bis(silylation) (also called double silylation) of
unsaturated organic substrates such as alkynes, olefins, iso-
cyanides, aldehydes and ketones. This elegant synthetic strategy
allows facile transformation of these substrates to organosilicon
compounds [8e12]. Notably, palladium catalysts such as
[Pd(PPh3)4], [PdCl2(PR3)2] or in situ generated [Pd(CNR)2] (obtained
by reduction of Pd(OAc)2 with isonitriles CNR) are currently used
for the catalytic bis(silylation) of these substrates [13e21]. Recent
theoretical investigations performed on the activation of SieSi
bond have revealed that oxidative addition is a facile process
with almost no energy barrier [22]. Furthermore, the frontier
orbitals (HOMO and LUMO) of the SieSi bond have been postulated
to possess similar properties to the C]C bond [23]. From
* Corresponding author. Tel.: þ33 3 81 66 62 70.
** Corresponding author.
E-mail
addresses:
sebastien.clement02@univ-montp2.fr
(S.
Clément),
michael.knorr@univ-fcomte.fr (M. Knorr), mail@carsten-strohmann.de (C. Strohmann).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.11.022

H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
263
a mechanistic point of view, three key steps are involved in the
2.2. Bis(silylation) of ethynylferrocene and 4-ethynyl-1,10-biphenyl
catalytic cycle:
The straightforward double silylation of a series of terminal
aromatic alkynes with the formation of olefins possessing a syn-
arrangement of the two SiCH2SPh groups around the C]C double
bond encouraged us to react 1 also with ethynylferrocene. This
choice was driven by the well-known redox activity of the ferro-
cenyl unit, a feature that appears interesting for potential applica-
tions of the resulting silylated olefin.
(i) oxidative addition of the SieSi bond across a metal centre,
(ii) insertion of the unsaturated substrate into a MeSi bond,
(iii) and finally reductive elimination producing the bis(silylated)
substrate, possessing in the majority of cases a syn-arrange-
ment of the two silyl substituents stemming from the disilane
precursor.
The bis(silylation) reaction was performed under the same
conditions as reported in our previous work [27,28]. A toluene
solution of 1 was reacted with ethynylferrocene in presence of
The stability of some reaction intermediates has allowed to
isolate species such as Pd(IV) bis-silyl complexes, which have even
been characterized by X-ray diffraction [24].
a
catalytic amount of 1,1,3,3-tetramethylbutylisocyanide and
In the context of our on-going interest on organosilicon deriv-
atives and transition metal silyl complexes [25], we have recently
investigated the coordination chemistry of silanes functionalized
by phenylthiomethyl groups [26]. We have also prepared 1,1,2,2-
tetramethyl-1,2-bis(phenylthiomethyl)disilane (1) and coordi-
nated 1 via its dithioether functions as chelate or bridging ligand to
a range of transition metals such as Pt, Re and Ru [27]. Furthermore
we have demonstrated that the catalytic cleavage of the SieSi bond
by Ito’s Pd(OAc)2/CNR catalyst allows addition across the triple
bond of phenylacetylene, p-tolylacetylene and (ꢀ)-4-ethynyl[2.2]
paracyclophane to afford the corresponding bis(silylated) olefins,
whose Z-stereochemistry has been confirmed by an X-ray diffrac-
tion study on Z-(PhSCH2)Me2SiC(H)]C(Ph)SiMe2(CH2SPh) [27,28].
In this work, we have extended our investigation on the palladium-
catalyzed double silylation of 1 across ethynylferrocene, since the
presence of an electrochemically active ferrocenyl moiety appeared
attractive for further studies. The characterization of the products
of the bis(silylation) reaction of ethynylferrocene and 4-ethynyl-
1,10-biphenyl by multinuclear NMR techniques is described along
with a re-examination of the coordination chemistry of 1 in the
presence of a stoichiometric amount of [PdCl2(PhCN)2], giving rise
to an unusual eight-membered Pd(II) chelate complex of compo-
sition cis-[PdCl2{(PhSCH2SiMe2)2O}].
palladium acetate (Scheme 1). The orange starting mixture turned
rapidly dark red-brown upon heating at 100 ꢁC. After one day, work
up was completed by column chromatography allowing the isola-
tion of pure 2 in 67% yield as an orange waxy solid. ESI-mass
spectrometry revealed the expected molecular mass peak at m/
z ¼ 573.1 corresponding to [M þ Hþ]. The 1H NMR spectrum of 2 in
CDCl3 displays two singlets at
d
¼ 0.41 and 0.48 ppm attributed to
the SiCH3 groups, the singlet resonances due to the methylene
SiCH2 protons appear at
¼ 2.34 and 2.43 ppm. The ferrocenyl
signals are present as a singlet at
¼ 4.20 ppm assigned to
unsubstituted Cp ring proton and two other unresolved signals at
d
d
d
¼ 4.24 and 4.44 ppm attributed to the substituted Cp ring (see
Fig. S1 in the Supporting information). As expected for two
unequivalent SiR3 groups, distinct resonances are observed in the
29Si{1H} NMR spectrum at
d
¼ ꢂ10.1 and ꢂ6.7 ppm (Fig. 2). The
biphenyl-substituted derivate Z-(PhSCH2)Me2SiC(H)]C(biphenyl)
SiMe2(CH2SPh) (3) was prepared in a similar manner and isolated
in 64% yield as a yellow waxy solid. The similarity of the 1H and 29Si
{1H} NMR data (see Experimental section) with those of 2 and Z-
(PhSCH2)Me2SiC(H)]C(Ph)SiMe2(CH2SPh) indicates again a cis-
arrangement around the vinylic double bond.
2.3. Synthesis of palladium complex 4
2. Results and discussion
The versatile coordination chemistry of ligand 1 across several
transition metal centres [Re(I), Ru(II), Pt(II)] has recently been
described by us [27]. In the case of cis-[PtCl2(PhCN)2], treatment
with 1 in technical dichloromethane has led at room temperature
straightforwardly to the seven-membered chelate complex cis-
[PtCl2{(PhSCH2)2Si2Me4}]. We have also reported that for cis-
[PdCl2(PhCN)2] under identical reaction conditions, we failed to
2.1. Crystal structure of 1,1,2,2-tetramethyl-1,2-
bis(phenylthiomethyl)disilane (1)
In order to better understand the reactivity of this versatile
ligand, we have determined the crystal structure of 1, which crys-
tallizes in form of huge colourless blocks having space group P21/n
within the monoclinic crystal system. The crystal structure is pre-
sented in Fig. 1, the crystal and refinement data are listed in Table 1.
In the centrosymmetric molecule, the SieSi bond distance is
isolate
a homologous complex cis-[PdCl2{(PhSCH2)2Si2Me4}].
Instead, precipitation of large amounts of elemental Pd was noticed
after some minutes. In light of some isolated Pd(II) and Pd(IV) silyl
complexes, this reaction was reinvestigated in more details. Thus,
[PdCl2(PhCN)2] was treated with ligand 1 in technical dichloro-
methane solution at room temperature (Scheme 2). The initial
orange solution turned rapidly to a dark mixture with slow
precipitation of colloidal palladium. After stirring overnight and
subsequent evaporation of all volatiles, extraction of the dark solid
with diethyl ether afforded an orange solid. Recrystallization from
somewhat shorter than in fac-[{RuCl2(CO)3}2{
m
-(PhSCH2)2Si2Me4}]
(2.3264(1) vs. 2.3533(12) A), where disilane bridges two
Ru(CO)3Cl2 fragments. Other examples of structurally characterized
disilanes containing SeCeSiMe2eSiMe2eCeS motif are
ꢀ
1
a
ꢀ
(TTF)2(Si2Me4) (TTF ¼ tetrathiafulvalene; 2.327(3) A) and macro-
cyclic 3,3,4,4,8,8,9,9-octamethyl-1,6-dithia-3,4,8,9-tetrasilacyclo-
ꢀ
decane (2.3450(4) A) [29]. There are neither intermolecular nor
a
cold
CH2Cl2/n-hexane
mixture
afforded
pure
cis-
intramolecular interactions between the sulphur atoms and the
[PdCl2{(PhSCH2SiMe2)2O}] (4) as orange crystals suitable for X-ray
crystallography, albeit in quite low yield (20%) [31].
silicon centres. The shortest intermolecular S/Si contact found in
ꢀ
the molecular cell is of 5.341 A. For 1-fluoro-1,2-dimethyl-2,2-
The IR spectrum recorded in KBr exhibits along with a strong
SieMe vibration at 1258 cmꢂ1 an additional intense band at
1029 cmꢂ1 corresponding to the SieOeSi group. This unexpected
presence of a siloxane unit was corroborated by an ESI-MS analysis
(positive mode), displaying the molecular peak [M]þ of complex 4
at m/z ¼ 556. The 1H NMR spectrum of 4 recorded at room
temperature exhibits quite broad resonances for the SiCH3, SCH2,
and SPh protons indicative for a dynamic behaviour in solution,
diphenyl-1-(8-phenylthio-1-naphthyl)disilane, an intramolecular
ꢀ
S/Si distance of 2.9668(6) A has been reported, which is consid-
erably shorter than the sum of the van der Waals radii between an S
ꢀ
and an Si atom (3.90 A) [30]. In the case of 1, the intramolecular
ꢀ
S/Si contact amounts to 3.016 A. However, the SeC7eSi angle of
109.7(11)ꢁ and C8eSieC9 angle of 110.91(12)ꢁ rule out any incipient
penta-coordination of Si as encountered in the former disilane.

264
H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
C3'
C9
C8
C4'
C2'
S
Si
C7'
C9'
C6
C5'
C1
C1'
C6'
C7
Si'
C5
C4
S'
C2
C8'
C3
ꢁ
0
ꢀ
Fig. 1. SCHAKAL plot of disilane 1. Selected bond lengths (A) and angles ( ). C(1)eC(2) 1.374(3), C(1)eS 1.768(2), C(7)eS 1.798(2), C(7)eSi 1.890(2), C(8)eSi 1.855(2), SieSi
2.3264(11), C(2)eC(1)eC(6) 119.60(19), C(2)eC(1)eS 123.75(16), C(6)eC(1)eS 116.65(16), SeC(7)eSi 109.70(11), C(1)eSeC(7) 104.22(10), C(8)eSieC(9) 110.91(12), C(9)eSieSi0
110.66(9). Symmetry transformations used to generate equivalent atoms: ꢂx þ 2, ꢂy þ 2, ꢂz þ 1.
which was monitored by variable-temperature 1H NMR. At 213 K,
resonances assigned to the SieCH3 groups appear as multiple
singlets in the range between 0.32 and 0.65 ppm. The signals due to
the methylene protons are more informative, since three AX
systems of different intensities are observed between 1.88 and
been determined for the inversion process occurring at the chelate
complex cis-[PdCl2(MeSCH2CH2SMe)] [32]. The temperature-
dependent coexistence of several invertomers has been evi-
denced by us in the case of the related six- and seven-membered
chelate
complexes
cis-[PtCl2{(PhSCH2)2SiPh2}]
and
cis-
2
3.59 ppm with JHeH couplings of 13 and 12 Hz, respectively. This
[PtCl2{(PhSCH2)2Si2Me4}] by means of 195Pt and 1H NMR at variable
temperature [26b,27]. But in case of complex 4, the particular
flexible SieOeSi backbone confers additional conformational
freedom. It is known, that even weakly coordinating siloxane
oxygen atoms can bind in an intramolecular manner to Pd(II) [35].
Therefore, the presence of an additional isomer may be associated
with a conformation involving a weak apical Pd/O contact. Indeed,
observation reveals the presence of at least three isomers in solu-
tion in a 1/1.2/1.6 ratio. This is certainly in part due to the well-
known pyramidal inversion of sulphur atoms occurring in square
planar platinum and palladium dithioether complexes. The work of
Abel et al. on this type of complexes has demonstrated that due to
this phenomenon, two isomers (invertomers) exist in solution,
which differ on the orientation of the SeR groups [32e34]. As
shown in Scheme 2, the SePh groups are cis-arranged (syn) in the
u-invertomer (meso-form), whereas they are trans-arranged (anti)
ꢀ
this kind of incipient penta-coordination (d Pd/O ¼ 2.968(3) A)
above the S2PdCl2 coordination plane of the complex cis-[PdCl2 (1-
oxa-4,7-dithiacyclononane)] has been evidenced by an X-ray study
[36]. Although the shortest intramolecular Pd/O contact amounts
in the l-invertomer. An activation energy
D
G# of 70.2 kJ molꢂ1 has
Table 1
Crystal and refinement data for compounds 1 and 4.
Compound
1
4
Empirical formula
C18H26S2Si2
362.69
173(2)
C18H26Cl2OPdS2Si2
555.99
173(2)
Formula weight [g molꢂ1
Temperature [K]
]
ꢀ
Wavelength [A]
0.71073
Monoclinic
P21/n (14)
10.2511(7)
9.9931(5)
10.4438(7)
111.570(8)
994.94(11)
2
0.71073
Monoclinic
P21/n (14)
8.6216(17)
13.412(3)
19.858(4)
91.90(3)
2295.0(8)
4
Crystal system
Space group (Nr.)
ꢀ
a [A]
ꢀ
b [A]
ꢀ
c [A]
b
[ꢁ]
3
ꢀ
Volume [A ]
Z
Density (calculated) [g cmꢂ3
]
1.211
0.383
388
1.609
1.334
1128
Absorption coefficient [mmꢂ1
F(000)
]
Crystal size [mm3]
0.20 ꢃ 0.10 ꢃ 0.10
0.30 ꢃ 0.20 ꢃ 0.10
Theta range for data collection [ꢁ]
Index ranges
2.38e27.00
2.55e25.99
ꢂ13 ꢄ h ꢄ 13
ꢂ10 ꢄ h ꢄ 10
ꢂ16 ꢄ k ꢄ 16
ꢂ24 ꢄ l ꢄ 24
21,171
ꢂ12 ꢄ k ꢄ 12
ꢂ12 ꢄ l ꢄ 13
Reflections collected
7953
Independent reflections
Refinement method
2174 (Rint ¼ 0.0374)
Full-matrix least-squares on F2
2174/0/102
4338 (Rint ¼ 0.0350)
Data/Restraints/Parameters
4338/0/239
1.008
Goodness-of-fit on F2
1.000
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole [e A
s
(I)]
R1 ¼ 0.0360, wR2 ¼ 0.0975
R1 ¼ 0.0536, wR2 ¼ 0.1007
0.393 and ꢂ0.313
R1 ¼ 0.0370, wR2 ¼ 0.0974
R1 ¼ 0.0417, wR2 ¼ 0.1056
0.582 and ꢂ0.877
ꢀꢂ3
]

H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
265
Scheme 1. Synthesis of the bis(silylated) olefins 2 and 3.
ꢀ
to 3.626 A in the solid-state structure of 4 (see below), other
inversion process without dissociation of a PdeS bond may operate
in parallel [32e34].
conformations allowing closer Pd/O contacts may be taken in
consideration. Concerning the resonances due to the Ph groups, it is
noteworthy that at low temperature a doublet at
arises whereas there is no signal below 7.26 ppm at 293 K. This
shielded resonance may result from conformer with two
perpendicular phenyl groups, such a conformation allowing the
ortho H atoms of one phenyl to point towards the other aromatic
cycle.
d
¼ 6.72 ppm
2.4. Crystal structure of complex 4
a
Fig. 4 shows the molecular structure of complex 4, which crys-
tallizes in the monoclinic crystal system, space group P2(1)/n
(Table 1). The Pd(II) centre is coordinated by two sulphur atoms and
two chlorine atoms in a square planar geometry. The two cis-
arranged Cl atoms form an Cl(1)ePdeCl(2) angle of 90.96(3)ꢁ, the
angle between the S atoms of 90.57(3)ꢁ also indicates no defor-
mation. The RMS deviation from best plane comprising Cl(1), Cl(2)
Taking the relatively high inversion barrier of three-coordinate
sulphur atoms into account, two diastereomeric invertomers have
to be considered in case of a stereochemically active lone pair
(Fig. 3). Therefore, we performed DFT calculations on the B3LYP/6-
31 þ G(d) level regarding the two most plausible conformational
isomers l-4 and u-4 (meso-invertomer), based on the molecular
structure of 4 in the crystal that is exclusively found as the l-form
(Fig. 4). The energetic gap between the two optimized invertomers
accounts for only 13 kJ molꢂ1 with l-4 being the energetically more
stable invertomer. The calculations approve the coexistence of the
two invertomers in solution in consequence of an interconversion
equilibrium that presumably proceeds via an open form of 4.
Quantum chemical studies support the temporary appearance of an
intermediate (4-Int) that is 85 kJ molꢂ1 higher in energy than l-4
with a disconnected “hemi-labile” PdeS bond and the siloxane
oxygen atom now coordinating to the palladium centre, leading to
a five-membered ring (Fig. 3, right). After rotation and reconnection
of the sulphur atom to palladium, inversion and hence formation of
the respective diastereomer is completed. Of course the classical
ꢀ
Pd, S(1) and S(2) amounts to 0.0343 A. Within the dithioether
ligand, the oxygen atom is inserted between the two silicon atoms
of the disilane backbone of 1 forming a Si(1)eOeSi(2) angle of
144.18(15)ꢁ. The planes of the two phenyl groups adopt a nearly
perpendicular orientation and are anti-arranged corresponding to
the l-invertomer. The eight-membered ring adopts a boat-like
conformation
[C(1)eS(1)ePd
110.53(10)ꢁ,
C(2)eS(2)ePd
107.50(11)ꢁ]. The PdeS(1) and PdeS(2) bond length of 2.3089(9)
ꢀ
and 2.2974(8) A are somewhat longer than the mean bond lengths
ꢀ
of related chelate complexes cis-[PdCl2{PhS(CH2)3SPh}] [2.284 A]
ꢀ
and cis-[PdCl2{PhS(CH2)2SPh}] [2.263 A] [37]. The average PdeCl
ꢀ
bond length [2.31755 A] lies in the usual range found for four-
ꢀ
coordinate Pd(II) complexes [2.298e2.354 A] [38]. The intra-
molecular Pd/O separation of 3.626 A is far too remote even for
a loose contact.
ꢀ
The surprising formation of a Pd(II) complex chelated by an
eight-membered ring is quite unusual and deserves some
comments. First of all, there are only very few precedents in the
literature on transition metal complexes forming eight-membered
chelates with acylic donor ligands. This bonding mode is less
common compared to the plethora of five-, six-, and seven-
membered systems due to the steric constraints imposed by an
eight-membered cycle. Among the rare examples of eight-
membered chelate complexes [39], there is the 1,5-
bis(diphenylphosphino)pentane ligand, which coordinates to
[PdCl2(NCPh)2] yielding cis-[PdCl2{Ph2P(CH2)5PPh2}] [40]. Inter-
estingly, the latter chelate complex undergoes upon heating a CeH
activation reaction of the central methylene group forming the
cyclometallated complex [PdCl{PPh2CH2CH2CHCH2CH2PPh}] [40].
This creation of a PdeC bond confirms our hypothesis, that in the
case of 4 the siloxane O-atom can come close enough to Pd(II) to
form species (4-Int) featuring a weak dative PdeO bond as shown in
Fig. 3. In the case of dithioether ligands, there is a report on the
photochemically-induced complexation of 1,5-bis(tert-butylthio)
pentane on W(CO)6 to afford cis-[W(CO)4{tBuS(CH2)5StBu}] [41].
Fig. 2. 29Si{1H} NMR spectrum in CDCl3 at 298 K of ferrocenyl compound 2.

266
H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
Scheme 2. Synthesis of complex 4.
However, addition of the ligand 1,5-bis(tert-butylthio)pentane to
[PdCl2(NCPh)2] produces the mesocyclic 16-membered dinuclear
complex trans-[Pd2Cl4{tBuS(CH2)5StBu}2] [42]. The ligand 1,5-
bis(phenylthio)pentane, which is quite comparable to ligand 1,
coordinates to [PdCl2(NCPh)2] forming the 1D coordination polymer
trans-[PdCl2{PhS(CH2)5SPh}]n [43]. This contrasting behaviour in
the coordination of the latter dithioether and ligand 1 upon
complexation on Pd(II) could probably be rationalized by the higher
conformational flexibility of the CeSieOeSieC spacer compared to
a CeCeCeCeC spacer, such diminishing steric constraints in the
cycle. Furthermore, coordination of the central siloxane O-atom
giving first rise to an intermediate five-membered chelate complex
(see Scheme 2), which subsequently rearranges into an eight-
membered ring, may also explain the diverging coordination
modes between the 1,5-bis(phenylthio)pentane and (PhSCH2)
Me2SieOeSiMe2(CH2SPh) ligands.
Fig. 3. Optimized structures of the two diastereomeric invertomers l-4 and u-4 (left) and the five-membered interconversion intermediate 4-Int (right) [B3LYP/6-31 þ G(d)].

H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
267
silanol groups. Both the presence of the chlorosilane and disiloxane
has been confirmed by GC/MS experiments (see below). Finally, the
disiloxane-derived dithioether (PhSCH2)Me2SieOeSiMe2(CH2SPh)
coordinates on unreacted [PdCl2(PhCN)2] through its sulphur
atoms to account for the formation of eight-membered chelate
complex cis-[PdCl2{(PhSCH2SiMe2)2O}] (4).
C15
C14
C13
C16
C17
C6
C2
Our mechanistic suggestion is supported by the following facts
and observations:
Cl1
Si2
S2
C18
(i) As already mentioned above, silyl complexes of Pd(IV) have
been characterized by X-ray crystallography studies [24,44].
The formation of an intermediate Pd(IV) bis(silyl) complex has
also been postulated by Seyferth et al. to explain the formation
of ClSiMe2CMe2CMe2SiMe2Cl upon treatment of octamethyl-
C5
Pd
C7
O
Cl2
C8
Si1
l,2-disilacyclobutane with
[PdCl2(PPh3)2] [16d].
a
stoichiometric amount of
C9
(ii) To exclude the possibility that the SPh groups are involved
during the oxidative addition/reductive elimination steps, the
reaction of non-activated hexamethyldisilane with an equi-
molar amount of [PdCl2(PPh3)2] in technical-grade CH2Cl2 was
examined. Again, a rapid precipitation of colloidal Pd was
noticed. IR analysis revealed that formation of Me3SiOSiMe3
occurred. The formation of ClSiMe3 was evidenced in an
indirect manner by the liberation of HCl liberated during the
hydrolysis of ClSiMe3. Independently from our experiment,
the same result has been recently reported by Kashiwabara
and Tanaka, who conducted this reaction in toluene and
detected formation of ClSiMe3 by 29Si NMR within 5 min after
mixing the starting materials [45].
C1
S1
C4
C3
C12
C10
C11
ꢁ
ꢀ
Fig. 4. SCHAKAL plot of complex 4. Selected bond lengths (A) and angles ( ). PdeS(2)
2.2974(8), PdeS(1) 2.3089(9), Cl(1)ePd 2.3116(9), Cl(2)ePd 2.3235(9), OeSi(1)
1.640(2), OeSi(2) 1.644(2), C(1)eSi(1) 1.895(3), C(2)eS(2) 1.790(3), C(2)eSi(2) 1.903(3),
C(3)eSi(1) 1.850(3), C(7)eS(1) 1.791(3), Si(1)eOeSi(2) 144.18(15), S(2)ePdeS(1)
90.57(3), S(2)ePdeCl(1) 92.28(3), S(1)ePdeCl(1) 176.79(3), S(2)ePdeCl(2) 176.07(3),
S(1)ePdeCl(2) 86.25(3), Cl(1)ePdeCl(2) 90.96(3), C(7)eS(1)eC(1) 106.57(14), C(7)e
S(1)ePd 103.50(10), C(1)eS(1)ePd 110.53(10), C(2)eS(2)ePd 107.50(11), C(13)eS(2)e
Pd 108.94(10), OeSi(1)eC(4) 112.11(15), OeSi(2)eC(5) 110.37(14).
2.5. Discussion on the formation mechanism of complex 4
The reaction of disilane ligand 1 with [PdCl2(PhCN)2] in dry
dichloromethane at room temperature was also repeated and its
evolution monitored by GCeMS analysis through diluted sample
measurements. Typically, an analytical sample was collected from
We suggest the following mechanism to rationalize the unex-
pected formation of complex 4 bearing a chelating (PhSCH2Si-
Me2)2O ligand (Scheme 3). After initial coordination of the
dithioether on Pd(II) forming a seven-membered chelate analogous
to cis-[PtCl2{(PhSCH2)2Si2Me4}], oxidative addition of the SieSi
bond occurs to yield an labile Pd(IV) intermediate. This latter may
then get reduced via reductive elimination of chlorosilane ClSi-
Me2(CH2SPh), thus explaining the formation of elemental palla-
dium. Chlorodimethyl(phenylthiomethyl)silane in turn gets
hydrolyzed by small amounts of H2O present in technical grade
dichloromethane to give rise to a disiloxane by condensation of the
the reaction mixture (100
mL) and diluted with additional CH2Cl2
solution (100 L) prior to the injection. After a running time of
m
15 min, GCeMS analysis revealed a peak at m/z ¼ 103 assigned to
the decomplexation of benzonitrile from the Pd(II) centre and
a second peak at m/z ¼ 216, attributed to PhSCH2SiMe2Cl resulting
from reductive elimination of a Pd(IV) intermediate. Analysis of the
reaction mixture after 2 h showed a main peak at m/z ¼ 216 due to
the chlorosilane, the remaining benzonitrile ligand at m/z ¼ 103
Scheme 3. Supposed formation mechanism of compound 4.

268
H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
Scheme 4. Examples of some cyclic siloxanes.
and the disilane 1 at m/z ¼ 362. No significant evolution of the
of an oxygen atom into the SieSi bond of 1 has been mechanisti-
cally elucidated. Complex 4 has been alternatively prepared in an
improved yield by treatment of [PdCl2(PhCN)2] with the siloxane
(PhSCH2SiMe2)2O.
chromatogram was noticed after 24 h and 48 h. However, when
100
mL of a sample in dry dichloromethane solution was collected
and diluted with 100
mL of technical-grade dichloromethane, GCe
MS analysis indicated a new peak at m/z ¼ 379 assigned to the
siloxane (PhSCH2SiMe2)2O formed by intermolecular condensation
of two chlorosilane molecules in presence of water to give the
disiloxane (see Fig. S4 in the Supporting information).
4. Experimental section
4.1. General procedures
(iii) It is literature-known that oxygen insertion into a SieSi bond
leading to a siloxane may also occur by exposure to air,
peroxide, tertiary amine oxide or water [46e48]. For instance,
Seyferth et al. have reported the exothermic hydrolysis of
octamethyl-1,2-disilacyclobutane by adding deoxygenated
water to form almost quantitatively 2,2,3,3,4,4,5,5-octamethyl-
1-oxa-2,5-disilacyclopentane [16d]. Thermal treatment of 3,4-
carboranylene-1,1,2,2-teraethyl-1,2-disilacyclo-but-3-ene in
air-containingtoluenesolution resultedalso in the formationof
the cyclic siloxane 4,5-carboranylene-1,3-disila-2-oxacyclo-
pent-4-ene (Scheme 4) [49].
Reactions were performed in oven-dried Schlenk glassware
under an atmosphere of pure nitrogen when necessary. Solvents
were dried over molecular sieves and degassed prior to use. IR
spectra have been recorded on a Nicolet Nexus 470 spectrometer.
NMR spectra were recorded on Bruker Advance 300 spectrometers.
Chemical shifts d are given in ppm relative to tetramethylsilane and
coupling constants J are given in Hz. The GC/MS system consists of
a Shimadzu GC-2010 gas chromatograph with Solution software
2.10. The column was a 25 m long FS-OV1-CB-0.25 column with
0.25 mm ID and 0.36 mm film thickness with a temperature
maximum between 300 and 320 ꢁC. The carrier gas was 99.995%
pure helium used at a flow rate of 1.06 mL minꢂ1 (40.8 cm sꢂ1) with
a head pressure of 9.4 psi at 40 ꢁC. The sample in CH2Cl2 solution
was injected manually using a splitless mode and the injection
However, no conversion of [PtCl2{(PhSCH2)2Si2Me4}] in CH2Cl2
solution upon prolonged exposure to air was noticed. Furthermore,
when ligand 1 was treated with cis-[PdCl2(PhCN)2] in dry CH2Cl2
under O2 atmosphere, there was no evidence of the formation of
complex 4. Therefore, direct oxygen insertion into an initially
formed chelate complex [PdCl2{(PhSCH2)2Si2Me4}] can be excluded
to explain formation of 4.
volume was 1
mL. Temperatures of the injection port and mass
selective detector interface were set at 150 and 200 ꢁC, respectively.
The temperature gradient of the GC oven was programmed to be
initiated at 60 ꢁC for 2 min, then raised to 290 ꢁC at 10 ꢁC minꢂ1 and
held at the final temperature for 5 min. An electron impact (EI)
ionization mode with 1.5 kV absolute voltage detector was used.
(iv) The last step of the proposed reaction sequence shown in
Scheme 3, namely coordination of (PhSCH2SiMe2)2O on
[PdCl2(PhCN)2], was proven by independent synthesis of this
siloxane. This latter was prepared by treatment of dime-
thyldichlorosilane with one equivalent of LiCH2SPh [50a], and
subsequent hydrolysis of the resulting chlorosilane
PhSCH2SiMe2Cl [50b]. Indeed, coordination of this siloxane on
[PdCl2(PhCN)2] in CH2Cl2 solution afforded readily chelate
complex 4 in an improved yield of 61%.
4.2. General procedure for the synthesis of bis(silylated)
compounds 2 and 3
To a toluene solution (10 mL) containing 1 (181 mg, 0.50 mmol),
ethynylferrocene or 4-ethynylbiphenyl (0.70 mmol) and palladium
acetate (2 mg, 0.01 mmol) was added 1,1,3,3-tetramethylbutyliso-
cyanide (27.8 mg, 0.2 mmol). The yellow mixture quickly turned
dark red after heating to 100 ꢁC. Stirring was continued for 1 d, and
then all volatiles were removed under reduced pressure. The
residue was purified by column chromatography with CH2Cl2/n-
hexane (1:9) as eluent affording 2 or 3 in 67 and 64% yield,
respectively.
3. Conclusion
We have shown that Pd-catalyzed double silylation of disilane
ligand 1 can also be applied to functionalized terminal alkynes such
as ethynylferrocene to provide the alkenyl-dithioether Z-(PhSCH2)
Me2SiC(H)]C(Fc)SiMe2(CH2SPh) (2). Since ferrocenyl-dithioethers
represent interesting redox-active metalloligands [51], we are
currently exploiting the propensity of 2 to assemble polymetallic
system and coordination polymers. To enlarge the range of avail-
able alkenyl-dithioether ligand, we are currently probing the
addition of 1 across terminal aliphatic alkynes. We have further-
more shown in this study that oxidative addition of disilane 1
occurs also across [PdCl2(PhCN)2] even at ambient temperature
producing the unusual fluxional 8-membered dithioether complex
cis-[PdCl2{(PhSCH2SiMe2)2O}] (4). The unexpected formal insertion
2: Anal. Calcd for C30H36FeS2Si2 (572.1) C, 62.91; H, 6.34, S, 11.20.
Found: C, 62.78; H, 6.26, S, 11.01. 1H NMR:
d 0.41 (s, 6H, SiCH3), 0.48
(s, 6H, SiCH3), 2.34 (s, 2H, SiCH2SPh), 2.43 (s, 2H, SiCH2SPh), 4.20 (s,
5H, Cp), 4.24 (s, br, 2H, Cp), 4.44 (s, br, 2H, Cp), 7.13e7.19 (m, 2H, Ph),
7.29e7.42 (m, 9 H, Ph þ ]CH) ppm. 13C{1H} NMR:
d 0.0, 0.2 (SiCH3),
18.9, 19.8 (1C, SiCH2S), 68.0, 69.2, 70.1 (Cp), 125.0, 125.2, 126.6,
126.7, 129.1, 129.2, 140.5, 140.6, 145.8, 159.2 ppm. 29Si{1H} NMR:
d
ꢂ10.1 (s, 1 Si), ꢂ6.7 (s, 1 Si) ppm. ESI-MS (m/z): 573.1 (M þ H).
3: Anal. Calcd for C32H36S2Si2 (542.2) C, 71.05; H, 6.71, S, 11.86.
Found: C, 70.82; H, 6.51, S, 11.79. 1H NMR:
d
0.42 (s, 6H, SiCH3), 0.48
(s, 6H, SiCH3), 2.34 (s, 2H, SiCH2SPh), 2.41 (s, 2H, SiCH2SPh), 6.69 (s,
1H, ]CH), 7.10e7.68 (m,19H, Ph) ppm. 13C{1H} NMR:
ꢂ0.25, ꢂ0.19
d

H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
269
(SiCH3), 19.0, 19.2 (1C, SiCH2S), 125.2, 125.3, 126.3, 126.6, 126.7,
energies (ZPE) of the calculated systems can be found in Table SI 1.
The calculated standard orientations of the optimized molecules l-
4, u-4 and 4-Int are summarized in Tables SI 2eSI 4 in the Sup-
porting information.
126.8, 127.0, 128.1, 129.0, 129.1, 129.2, 139.4, 141.2, 143.9, 148.0,
149.0, 163.7 ppm. 29Si{1H} NMR:
ppm. ESI-MS (m/z): 541.1 (M þ H).
d
ꢂ9.84 (s, 1 Si), ꢂ6.83 (s, 1 Si)
4.3. Preparation of cis-[PdCl2{(PhSCH2SiMe2)2O}](4)
Acknowledgements
Method A. To a solution of [PdCl2(PhCN)2] (192 mg, 0.50 mmol)
in 10 mL of technical-grade dichloromethane was added 1 (182 mg,
0.50 mmol). The yellow solution turned rapidly brown and
precipitation of Pd was noticed. After stirring overnight, all volatiles
including liberated benzonitrile were removed under vacuo. The
dark solid was then extracted with several portions of diethyl ether
(3 ꢃ 5 mL) and separated from the resulting suspension by filtra-
tion. From the combined fractions, an orange solid was isolated.
This latter was then re-dissolved in 3 mL of CH2Cl2 and a n-hexane
layer was added. Orange crystals (57 mg, 20% yield) suitable for X-
We are grateful to the Deutsche Forschungsgemeinschaft and
the CNRS for financial support. J.O.B. thanks the Fonds der Chem-
ischen Industrie (FCI) for a doctoral scholarship.
Appendix A. Supporting information
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2012.11.022.
ray diffraction were obtained. Method B. To
a solution of
References
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H.W. Peindy N’dongo et al. / Journal of Organometallic Chemistry 724 (2013) 262e270
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