Synthesis, characterisation and complexation of phosphino disilenes

Article abstract of DOI:10.1039/c0dt00180e

The treatment of disilenide Tip₂SiSi(Tip)Li (1, Tip = 2,4,6-ⁱPr₃C₆H₂) with P-chloro phosphines affords the phosphino disilenes (2a-d; a: R = Ph, b: R = ⁱPr, c: R = Cy, d: R = ^tBu), which were characterised by multinuclear NMR spectroscopy for 2a-d and a single crystal X-ray diffraction study in case of 2c. As an alternate synthetic method, the diphenyl derivative 2a could also be prepared by reaction of LiPPh₂ with the thermally unstable iododisilene, Tip₂Si=Si(Tip)I (3), which in turn was obtained by oxidation of 1 with stoichiometric amounts of iodine. Providing the first example for a SiSi bond with an iodo functionality, disilene 3 was fully characterised by multinuclear NMR and X-ray diffraction. The thermal rearrangement of phosphine disilene 2avia a C-H insertion reaction yields the diastereomeric mixture of a 1-phospha-2,3-disilaindane 4. The structure of the cis-diastereomer of 4 was determined by X-ray diffraction. Finally, the synthesis of first transition metal complex of 2a and 2b by their coordination to the [Pd(PCy₃)] fragment is reported. The solid state structure of complex 5a reveals η²-coordination of the Si=Si bond of the phosphino disilene 2a with an intermediate bonding mode between π-complex and metallacyclopropane rather than coordination of the phosphino group. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00180e

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New Horizon of Organosilicon Chemistry
Guest Editor: Professor Mitsuo Kira
Tohoku University, Japan
Published in issue 39, 2010 of Dalton Transactions
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PERSPECTIVES:
Silylium ions in catalysis
Hendrik F. T. Klare and Martin Oestreich
Dalton Trans., 2010, DOI: 10.1039/C003097J
Kinetic studies of reactions of organosilylenes: what have they taught us?
Rosa Becerra and Robin Walsh
Dalton Trans., 2010, DOI: 10.1039/C0DT00198H
π-Conjugated disilenes stabilized by fused-ring bulky “Rind” groups
Tsukasa Matsuo, Megumi Kobayashi and Kohei Tamao
Dalton Trans., 2010, DOI: 10.1039/C0DT00287A
Organosilicon compounds meet subatomic physics: Muon spin resonance
Robert West and Paul W. Percival
Dalton Trans., 2010, DOI: 10.1039/C0DT00188K
HOT ARTICLE:
Novel neutral hexacoordinate silicon(IV) complexes with two bidentate monoanionic benzamidinato
ligands
Konstantin Junold, Christian Burschka, Rüdiger Bertermann and Reinhold Tacke
Dalton Trans., 2010, DOI: 10.1039/C0DT00391C
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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, characterisation and complexation of phosphino disilenes†
Marco Hartmann, Abdishakur Haji-Abdi, Kai Abersfelder, Peter R. Haycock, Andrew J. P. White and
David Scheschkewitz*
Received 21st March 2010, Accepted 1st June 2010
DOI: 10.1039/c0dt00180e
i
=
The treatment of disilenide Tip2Si Si(Tip)Li (1, Tip = 2,4,6- Pr3C6H2) with P-chloro phosphines
i
t
affords the phosphino disilenes (2a-d; a: R = Ph, b: R = Pr, c: R = Cy, d: R = Bu), which were
characterised by multinuclear NMR spectroscopy for 2a-d and a single crystal X-ray diffraction study
in case of 2c. As an alternate synthetic method, the diphenyl derivative 2a could also be prepared by
=
reaction of LiPPh2 with the thermally unstable iododisilene, Tip2Si Si(Tip)I (3), which in turn was
obtained by oxidation of 1 with stoichiometric amounts of iodine. Providing the first example for a
=
Si Si bond with an iodo functionality, disilene 3 was fully characterised by multinuclear NMR and
X-ray diffraction. The thermal rearrangement of phosphine disilene 2a via a C–H insertion reaction
yields the diastereomeric mixture of a 1-phospha-2,3-disilaindane 4. The structure of the
cis-diastereomer of 4 was determined by X-ray diffraction. Finally, the synthesis of first transition metal
complex of 2a and 2b by their coordination to the [Pd(PCy3)] fragment is reported. The solid state
2
=
structure of complex 5a reveals h -coordination of the Si Si bond of the phosphino disilene 2a with an
intermediate bonding mode between p-complex and metallacyclopropane rather than coordination of
the phosphino group.
examples of genuine p-coordination of a Si–Si double bond (E)
Introduction
were observed by Kira and Iwamoto et al. in the complexes
comprising the 12-electron transition metal fragment, [Cy3PPd].9
We therefore considered phosphino-substituted disilenes attrac-
tive synthetic targets. They could reasonably be expected to exhibit
Vinyl-substituted phosphorus derivatives find applications in
organic synthesis and catalysis.1 Vinyl phosphines (A) are iso-
electronic neutral versions of the anionic allyl ligands and as
such have attracted considerable interest as precursors for various
complexes featuring the related anionic phosphaallyl ligands.2 The
coordination modes of neutral vinyl phosphines on the other hand
3
a versatile coordination behaviour including h -allyl type bonding,
2
hemilabile modes, and h -modes effected by coordination of
=
the Si Si bond with either p-complex or metallacyclopropane
3
range from the usual h -mode of the isomeric all-carbon analogue
character.
1
(B),3 to h -coordinating ligands,4 where the C–C double bond
In general, however, the synthesis of disilenes with hetero-
substitution is a non-trivial task. Until recently only a few exam-
ples have been reported, none of which concerned phosphorus.10
Besides, these procedures are mainly based on the dimerisation
of silylene fragments and as such unsuitable for the selective
introduction of just one hetero-substituent. Two methods emerged
in the meantime that allow for the selective incorporation of one
functional substituent. The first is based on addition chemistry
to the Si–Si triple bond of a disilyne granting a straightforward
access to amino and boryl-substituted disilenes.11 The second
method relies on formal nucleophilic substitution reactions by
disila analogues of vinyl anions, i.e. disilenides, which afforded
zirconium, boron, and tin-substituted derivatives.12
Here, we report that treatment of lithium disilenide 113 with the
appropriate P-chloro phosphines indeed provides a convenient
access to the first phosphino disilenes 2a–d. In addition, in case
of 2a, we report on an alternate synthetic approach employing
an umpolung strategy and its isomerisation upon heating. For 2a
and 2b, first transition metal complexes are reported, prepared by
reaction with [Pd(PCy3)2] as an electron-poor transition metal
fragment. All compounds were characterised by multinuclear
NMR spectroscopy in solution; solid state structures were con-
firmed by X-ray diffraction on single crystal of one representative
example of each class of compounds.
can act as a hemilabile donor protecting a vacant coordination
site (C).5
Despite sharing the same group of the periodic table with
carbon, silicon is—in terms of electronegativity and size—much
more akin to the group 15 element phosphorus. The coordination
behaviour of disilenes is therefore, though qualitatively similar
(Dewar–Chatt–Duncanson model),6 quantitatively very different
from that of alkenes. The higher HOMO and lower LUMO of
the Si–Si double bond7 account for both stronger s-donation to
the metal and stronger p-back donation from the metal to the
ligand, which explains the prevalence of the metallacyclopropane
binding mode in most disilene complexes (D).8 In fact, only two
Department of Chemistry, Imperial College London, Exhibition Road,
South Kensington, London, SW7 2AZ, UK. E-mail: d.scheschkewitz@
imperial.ac.uk; Fax: +44 (0)20 7594 5804; Tel: +44(0)20 7594 7473
† CCDC reference numbers 768711 (2c), 768712 (3), 768713 (4), and
769675 (5). For crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0dt00180e
9288 | Dalton Trans., 2010, 39, 9288–9295
This journal is
The Royal Society of Chemistry 2010
©

CH3, P-iPr-CH3). 29Si NMR (C6D6) d = 96.6 (br, 2J (Si,P) = ca.
60 Hz, SiC2), 53.2 (1J (Si,P) = 118 Hz, SiCP). 31P NMR (C6D6)
d = -21.9 (v br).
Experimental
General
All manipulations were carried out in oven-dried glassware
under an atmosphere of argon or nitrogen using Schlenk-type
techniques or in a glovebox. Solvents were dried and freed
of oxygen by reflux for at least two days in the presence of
the appropriate reagent. Disilenide 1 was synthesized with two
coordinating molecules of 1,2-dimethoxyethane according to our
published procedure.13 Chlorophospines were purchased from
Sigma–Aldrich and distilled tube-to-tube under argon prior to
use. NMR spectra were recorded at room temperature on a Bruker
Avance 500 (1H: 500.13 MHz, 13C: 125.77 MHz, 29Si: 99.37 MHz,
31P: 202.46 MHz). Chemical shifts are reported relative to SiMe4
(1H, 13C, 29Si) and H3PO4, 85% (31P, external). UV/vis spectra
were recorded on a Shimadzu UV Mini 1240 photometer. Melting
points were determined in closed NMR tubes under argon and are
uncorrected. Microanalyses on crystallised samples on sufficient
spectroscopic purity were carried out with a vario MICRO
elemental analyser from Elementaranalysensysteme GmbH.
1-Dicyclohexylphosphino-1,2,2-tris(2¢,4¢,6¢-triisopropylphenyl)-
disilene, 2c. Orange crystals from hexane (0.23 g, 23%, mp. 140
◦
1
C). H NMR (C6D6) d = 7.09 (br), 6.98, 6.97 (br) (each s, 6H,
i
Tip-H), 4.58 (br), 3.99, 3.74 (br, 6H, Pr–CH), 2.73, 2.72, 2.67
i
(each hept, 3H, Pr–CH), 2.10 (br), 1.66 (br), 1.46 (br), 1.42 (br)
(20H, Cy-CH2), 1.24, 1.19 (br), 1.16, 1.14, 1.11, 0.96 (br) (each
d, together 54H, iPr-CH3). 13C NMR (C6D6) d = 155.66, 155.23,
154.34, 150.83, 150.74, 150.53 (Tip-Co/p) 136.38 (br, d, J(C,P) =
10.96 Hz, Tip-Ci), 134.22 (br, Tip-Ci), 132.96 (d, J(C,P) = 1.94 Hz,
Tip-Ci), 122.42, 122.16 (br) (Tip-CH), 37.92, 37.62 (br), 34.73,
34.68, 34.50 (iPr-CH), 33.24, 27.88 (br), 26.63 (Cy-CH2), 25.80
(br), 24.49 (br), 24.11, 24.05, 23.99 (iPr-CH3, Cy-CH2). 29Si NMR
(C6D6) d = 95.4 (br, 2J (Si,P) = ca. 60 Hz, SiC2), 52.5 (br.,
1J (Si,P) = ca. 115 Hz, SiCP).31P NMR (C6D6) d = -36.3 (v
br). UV/Vis (hexane) lmax, nm (e, M-1cm-1) = 416 nm (18970).
Microanalysis calc. for C57H91Si2P: C, 79.30, H, 10.60. found.: C,
79.30, H, 10.90.
Synthesis
1-Di-tert.-butylphosphino-1,2,2-tris(2¢,4¢,6¢-triisopropyl-phenyl)-
disilene, 2d. orange-yellow solid. H NMR (C6D6) d = 7.28,
1
General procedure for the preparation of phosphino disilenes,
2a–d. At room temperature 1.17 mmol of the appropriate
chlorophosphane were added dropwise to a solution of 1.00
g (1.17 mmol) of disilenide 1 in 10 mL of toluene. Stirring
was maintained for 4 h before all volatiles were removed in
vacuum. The residue was digested with 20 mL of pentane and
LiCl separated by filtration. Removal of the solvent afforded the
product in NMR spectroscopic purity.
7.10, 7.04, 7.02, 6.93, 6.90 (each s, 6H, Tip-H), 4.77, 4.54, 4.03,
3.94, 3.55 (br, 6H, iPr–CH), 2.76, 2.72, 2.67 (each hept, together
3H, iPr–CH), 1.94 (br), 1.58 (br), 1.40 (br),1.36 (d), 1.30 (d), 1.19
(d), 1.15 (d), 1.11(d), 0.51, 0.44, 0.38 (together 72H, CH3). 13C
NMR (C6D6) d = 155.82, 155.39, 155.14, 155.05, 154.1, 153.92
(br, Tip-Co) 150.89, 150.61, 150.37 (Tip-Cp), 137.3 (J(C,P) =
15.3 Hz, Tip-Ci), 134.5 (J(C,P) = 1.7 Hz Tip-Ci), 133.2 (J(C,P) =
4.0 Hz Tip-Ci), 123.17, 123.09, 122.98, 122.06, 121.79, 121.50
(br, Tip-CH), 38.88, 38.73, 37.40, 37.02, 36.49, 36.93 (o-iPr-CH),
34.68, 34.67, 34.47 (p-iPr-CH), 33.61, 33.00 (d, 2J(C,P) = 15 Hz,
1-Diphenylphosphino-1,2,2-tris(2¢,4¢,6¢-triisopropyl-phenyl)dis-
ilene, 2a. Orange crystals from hexane (0.46 g, 46%, mp. 135
◦
1
C). H NMR (C6D6) d = 7.60 (t, 4H, Ph-H), 7.03 (s), 7.02 (s),
7.01, 7.00, 6.98 (m), 6.97, 6.93 (m) (6H, Tip-H and 6H, Ph-H),
4.27 (br), 3.95 (br), 3.86 (br), 2.78, 2.70, 2.68 (each hept, together
t
tBu–CH3), 34.10, 33.90 (2J(C,P) = 15 Hz, Bu–Cq), 29.72, 26.40,
25.97, 24.97, 24.71, 24.29, 24.17, 24.00 (br) (iPr-CH3). 29Si NMR
(C6D6) d = 98.8 (1J(Si,Si) = 143 Hz, 2J(Si,P) = 89 Hz, SiC2), 53.0
i
9H, Pr-CH), 1.26, 1.19, 1.14, 1.11, 0.97 (each d, together 54H,
1
(1J(Si,Si) = 143 Hz, J(Si,P) = 133 Hz, SiCP).31P NMR (C6D6)
iPr-CH3). 13C NMR (C6D6) d = 156.16, 155.58, 154.61, 151.28,
151.26 150.93 (Tip-Co/p), 138.13 (br, d, J(13C,31P) = 20.3 Hz, Ph-
CH), 134.25 (br, d J(C, P) = 18.0 Hz, Ph-CH), 127.43 (Ph-CH),
135.48 (d, J(C, P) = 15.0 Hz, Tip-Ci), 133.22 (Tip-Ci), 131.48 (d,
J(13C, 31P) = 3.0 Hz, Tip-Ci), 122.42, 122.35, 121.95 (Tip-CH),
38.74, 38.37, 37.73 (br), 34.74, 34.72, 34.50 (iPr-CH), 24.95 (br),
24.51, 24.11, 24.03, 23.95 (iPr-CH3). 29Si NMR (C6D6) d = 96.5
(2J(Si,P) = 77 Hz, SiC2), 54.4 (1J(Si,P) = 116 Hz, SiCP). 31P NMR
(C6D6) d = -45.7 (1J(Si,P) = 116 Hz, 2J(Si,P) = 77 Hz). UV/Vis
(hexane) lmax, nm (e, M-1cm-1) = 423 nm (18380). Microanalysis
calc. for C57H79Si2P: C, 80.41, H, 9.40. found.: C, 80.04, H, 9.50.
d = 8.9 (1J(Si,P) = 133 Hz, 2J(Si,P) = 89 Hz).
Synthesis
of
1-iodo-1,2,2-tris(2¢,4¢,6¢-triisopropyl-phenyl)-
disilene, 3. At -80 ◦C a solution of 0.31 g (1.17 mmol) iodine
in 10 mL of toluene was added dropwise to 1.00 g (1.17 mmol)
of disilenide 1 in 25 mL of hexane. The purple reaction mixture
was brought to room temperature within the course of 2h, during
which the colour of the solution changes to orange-red and
lithium iodide precipitates. The salts were filtered off and all
volatiles removed in vacuum. The residue was dissolved in a
◦
minimum amount of hexane. Storing at 0 C overnight afforded
1
1-Diisopropylphosphino-1,2,2-tris(2¢,4¢,6¢-triisopropyl-phenyl)-
disilene, 2b. orange-yellow solid. 1H NMR (C6D6) d = 7.08, 6.98,
6.97 (each s, 6H, Tip-H), 4.03 (br), 2.734, 2.728, 2.73, 2.67 (each
thermally sensitive orange crystals of 3 (0.12 g, 17%). H NMR
(C6D6) d = 7.10, 7.07,6.98 (each s, each 2 H, Tip-H), 4.40,
4.00, 3.76, 2.74 (br), 2.65 (br) (each hept, together 9H, iPr–CH),
1.38 (br), 1.3 (br), 1.24, 1.18, 1.17, 1.1, 0.96 (each d, together
hept, together 9H, Pr–CH), 2.26 (hept, 2H, P-iPr–CH), 1.61,
i
1.36, 1.24, 1.19 (br), 1.17, 1.11 (each d, iPr-CH3), 1.15 (d, P-iPr–
CH3). 13C NMR (C6D6) d = 155.57, 155.24, 154.32, 150.88, 150.78,
150.56 (Tip-Co/p), 136.3 (J(C, P) = 11.1 Hz, Tip-Ci), 134.11(Tip-
Ci), 132.9 (J(C,P) = 3.0 Hz, Tip-Ci), 122.29, 122.16 (br) (Tip-CH),
38.0, 37.67 (br), 34.75, 34.68, 34.50 (iPr-CH), 25.71, 25.25 (br),
24.42 (br), 24.09, 24.00 (iPr-CH3), 22.97 (v br, P-iPrCH, Tip-iPr-
54H, Pr-CH3). 13C NMR (C6D6) d = 155.79, 155.36, 154.20,
i
152.64, 151.83, 151.16 (Tip-Co/p) 135.56, 132.72, 132.68 (Tip-Ci),
122.41, 122.38, 122.11 (Tip-CH), 39.36, 38.52, 38.36, 34.86,
34.73, 34.57 (iPr-CH), 25.71 (br), 24.51, 24.17 (br), 24.10, 24.04,
23.92 (iPr-CH3). 29Si NMR (C6D6) d = 76.8 (SiC2) (1J(Si/Si) =
128 Hz), 51.7 (SiCI) (1J(Si/Si) = 128 Hz).
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9288–9295 | 9289
©

◦
n
Reaction of iodo disilene 3 with Ph2PLi. At 0 C BuLi (156
mL, 1.5 M in hexane) were added to a solution of 46.9 mg Ph2PH
(prepared by reaction of Ph2PCl with LiAlH4 in Et2O followed by
cautious quenching with degassed H2O and distillation) in 5 mL
of Et2O. After stirring 2 h at room temperature the resulting pale
yellow solution was added at -90 ◦C to a solution of 200 mg
(0.25 mmol) of 3 in 10 mL of Et2O. The reaction mixture was
warmed to room temperature and all solvents were distilled off in
vacuum. 1H and 31P NMR data: see 2a. 29Si NMR (C6D6) d = 96.5
(2J(Si,P) = 77 Hz, SiC2), 54.4 (1J(Si,P) = 116 Hz, SiCP), -41.4
(impurity), -69.1 (impurity).
g2-[1-Diphenylphosphanyl-1,2,2-tris(2¢,4¢,6¢-triisopropyl-phenyl)-
disileno]-tricyclohexylphospino-palladium(0), 5a. 1H NMR
(C6D6) d = 8.07 (br), 7.44 (br), 7.04, 7.02, 6.98 (pseudo-t), 5.16,
4.47, 4.16, 4.12, 3.99, 3.72, 2.78, 2.77, 2.72 (each hept, each
1H), 1.92 (br m), 1.74 (br m), 1.67, 1.64, 1.41, 1.35, 1.26, 1.25,
1.23, 1.21, 1.18, 1.17 (each d), 1.10 (br m), 0.99, 0.90, 0.88, 0.56,
0.55, 0.50, 0.42, 0.42 (each d). 13C NMR (C6D6) d = 157.08,
154.88, 153.19, 152.89, 152.56, 149.69, 149.32, 148.96, 141.34,
138.77 (br), 137.9 (d), 133.32 (br) 132.51, 129.26 (br), 125.78
(br), 122.69, 122.61, 122.21, 122.12, 121.97, 121.71, 40.48, 37.78,
37.68, 37.56, 37.49, 37.06, 36.60, 34.71, 34.53, 33.52, 33.46,
32.25, 32.18, 31.71, 31.58, 31.36, 31.33, 28.68, 28.16, 28.02, 27.92,
27.82, 26.93, 26.50, 26.45, 26.01, 25.10, 25.03, 24.62, 24.26, 24.12,
24.04, 23.99, 23.74, 23.41.29Si NMR (C6D6, RT) d = 43.9 (dd, 2J
(Si,P) = 29 Hz, 2J (Si,P) = 13 Hz), -3.3 (dd, 1J (Si,P) = 105 Hz,
2J (Si,P) = 23 Hz). 31P NMR (C6D6, RT) d = 22.5 (d, 3J(P,P) =
3 Hz), 10.0 (PCy3), -33.2 (d, 3J(P,P) = 3 Hz, 1J(P,Si) = 105 Hz).
UV/Vis (hexane) lmax, nm (e, M-1cm-1) = 491 nm (3800), 443
(4500).
Thermolysis of 2a. Isolation of a diastereomeric mixture of
1-phenyl-2,2,3-tris(2¢,4¢,6¢-triisopropylphenyl)-1-phospha-2,3-di-
silaindane, 4. A solid sample of 100◦mg (0.117 mmol) of 2a was
melted under argon and kept at 150 C for 1 h. After cooling, a
minimum amount of hexane was added and the sample kept at
room temperature overnight yielding colourless crystals suitable
for X-ray analysis (68 mg, 68%). In solution two diastereomers are
observed in a 72 : 28 ratio. 1H NMR (C6D6) d = 7.97 (t), 7.87, 7.85,
7.74, 7.52, 7.48, 7.26, 7.14, 7.12, 7.05, 6.94, 6.81, 6.77, 6.72, 6.55
(m, Ar-H), 6.01 (s, J(Si,H) = 192 Hz, J(Si,H) = 39 Hz, Si-H,
cis-4), 5.64 (s, 1J(Si,H) = 191 Hz, 2J(Si,H) = 40 Hz Si-H, trans-4),
4.55, 4.41, 4.17, 3.73, 3.41, 3.16, 2.79, 2.68, 2.41 (each hept., iPr–
CH), 1.47, 1.63, 1.57, 1.53, 1.49, 1.42, 1.39, 1.25, 1.19, 1.14, 1.10,
0.98, 0.93, 0.88, 0.67, 0.57, 0.53, 0.30, 0.28, 0.21 (iPr-CH3). 13C
NMR (C6D6, RT) d = d = 158.13, 157.15, 155.94, 155.55, 155.05,
154.68, 154.38, 154.16, 153.28, 151.13, 150.5, 150.32, 150.18,
147.28, 143.43, 143.34, 135.12, 133.63, 126.08 (Ar–C), 137.84,
137.75, 137.58, 137.43, 136.43, 135.44, 135.28, 135.12, 132.91,
132.79, 130.05, 126.24, 123.58, 123.40, 123.20, 122.85, 122.70,
122.40, 121.40 (Ar–CH), 38.08, 37.07, 36.06, 35.81, 35.76, 35.54,
35.22, 35.16, 34.77, 34.68, 34.63, 34.57, 34.01, 33.77, 31.97, 30.24,
29.03, 28.25, 27.61, 26.94, 25.82, 25.52, 25 40, 25.25, 25.04, 24.77,
24.46, 24.33, 24.11, 24.01, 23.95, 23.86, 23.55, 23.45, 23.27, 23.07,
22.90, 22.15 (iPr-CH3). 29Si NMR (C6D6) d = -8.6 (1J (Si,P) =
49 Hz, SiP, trans-4), -27.0 (1J (Si,P) = 35 Hz, SiP, cis-4), -28.2 (2J
(Si,P) = 20 Hz, trans-4), -40.4 (2J (Si,P) = 2 Hz, cis-4). 31P NMR
(C6D6) d = -41.2 (1J(Si,P) = 35 Hz), cis-4), -58.5 (1J(Si,P) =
49 Hz, 2J(Si,P) = 20 Hz, trans-4).
g2-[1-Diisopropylphosphanyl-1,2,2-tris(2¢,4¢,6¢-triisopropyl-phe-
nyl)disileno]-tricyclohexylphospino-palladium(0), 5b. 29Si NMR
(C6D6, RT) d = 48.0 (dd, 1J(Si/Si) = 90 Hz, 2J (Si,P) = 31 Hz, 2J
(Si,P) = 13 Hz), 1.1 (dd, 1J(Si/Si) = 90 Hz, 1J (Si,P) = 105 Hz, 2J
(Si,P) = 22 Hz). 31P NMR (C6D6, RT) d = 21.9 (s), 10.0 (PCy3),
-21.5 (s, 1J(P,Si) = 105 Hz).
1
2
X-ray structure determination
The solid state structures of 2c, 3, and cis-4† (Table 1) were
determined on a Bruker SMART Apex II diffractometer equipped
with a CCD area-detector and a custom-made cooling device;
the structure of 5a on an Oxford diffraction Xcalibur PX Ultra.
Absorption corrections based on symmetry equivalent reflections
were applied using SADABS.14 The structures were solved using
the SHELXTL program package15 by Direct Methods (2c, 3, 5a)
or Patterson synthesis (4). Dynamic or static disorder phenomena
were observed in lateral sections of the molecules and accounted
for as follows. One cyclohexyl-group of 2c (C52 to C57) was
disordered over two positions (ca. 82% and 18% occupancy)
and anisotropically refined without restraints. One para-isopropyl
groups of 3 (C40 to C42) was refined anisotropically on split
positions with restraints applied to (idealised) geometries and
the thermal parameters of neighbouring atoms. The same group
(C40, C41, C42) showed disorder in case of 4 and was refined
anisotropically on split positions with restraints to thermal
parameters only. In 5a, the included hexane solvent molecule was
found to be disordered about a centre of symmetry. Two unique
orientations of ca. 36% and 14% occupancy were identified (two
further orientations are generated by the centre of symmetry),
their geometries were optimised, and all of the atoms were
refined isotropically with the thermal parameters of neighbouring
atoms restrained to be similar. All other non-hydrogen atoms
(2c, 3, 4, 5a) were assigned anisotropic displacement parameters
and refined without positional constraints. All hydrogen atoms
except for H1 in 3, which was located on the difference elec-
tron density map and isotropically refined, were constrained to
ideal geometries and refined with fixed isotropic displacement
parameters.
Thermolysis◦of 2c. A sample of 100 mg (0.12 mmol) of 2c was
heated to 140 C for 1 h. After cooling to room temperature the
sample was analysed by NMR spectroscopy. 29Si NMR (C6D6)
d = 7.1 (1J (Si,P) = 91 Hz, minor isomer), -8.5 (2J (Si,P) = 14 Hz,
major isomer), -46.2 (1J (Si,P) = 62 Hz, major isomer), -59.2 (2J
(Si,P) = 13 Hz, minor isomer). 31P NMR (C6D6) d = -44.1 (minor
isomer), -56.0 (1J(Si,P) = 62 Hz, major isomer).
General procedure for the synthesis of phosphino disilene com-
plexes 5a,b. A mixture of 0.3 mmol of [Pd(PCy3)2] and 0.3 mmol
of the appropriate phosphino disilene was dissolved in 10 mL of
benzene and stirred at room temperature leading to a gradual
darkening. The reaction was complete after 18 h at the indicated
temperature (5a: 20 ◦C; 5c: 50 ◦C) affording a 1 : 1 mixture of
free PCy3 and 5a or 5b, respectively. The solvent was distilled
off in vacuum. Complex 5a was crystallised from hexane as red-
orange crystals suitable for X-ray diffraction in 10% isolated
yield.
9290 | Dalton Trans., 2010, 39, 9288–9295
This journal is
The Royal Society of Chemistry 2010
©

Table 1 Data of structure determinations by single crystal X-ray diffraction
Compound
2c
3
4
5a·0.5(C6H14)
Formula
C57H91PSi2
863.45
Monoclinic
P21/c
20.8212(12)
10.5035(6)
26.2637(16)
90
103.603(1)
90
5582.6(6)
4
1.027
173
25.03
1904
0.71073
0.125
42316
C45H69ISi2
793.08
Monoclinic
P21/c
23.6013(19)
10.7701(9)
17.9227(14)
90
98.941(2)
90
4500.4(6)
4
1.171
173
25.03
1680
0.71073
0.792
43109
C57H79PSi2
851.35
Monoclinic
P21/n
13.5451(9)
20.3454(13)
19.0323(12)
90
97.780(1)
90
5196.7(6)
4
1.088
173
25.03
1856
0.71073
0.134
40563
C78H119P2PdSi2
1281.25
Monoclinic
P21/c
19.87013(7)
14.06651(4)
27.06129(8)
90
100.9683(3)
90
7425.55(4)
4
1.146
173
72.58
2764
1.54184
3.016
90750
M/g mol-1
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
r/g cm-3
T/K◦
qmax
/
F(000)
˚
l/A
m/mm-1
Reflections Collected
Unique
9822
7936
9158
14685
Observed [I > 2s(I)]
Restraints
9336
0
7337
12
8432
32
13722
97
Parameters
Rint
R1 [I > 2s(I)]
wR2 (all data)
GOF on F2
614
481
589
771
0.0346
0.0839
0.1694
1.283
0.0203
0.0345
0.0846
1.049
0.0330
0.0645
0.1479
1.209
0.0257
0.0268
0.0714
1.061
-3
˚
Largest Diff. Peak, Hole/e A
0.701/-0.596
0.756/-0.487
0.469/-0.355
0.340/-0.422
Table 2 29Si and 31P NMR data of disilenes phosphine disilenes 2a–d,
iododisilene 3, and complexes 5a,b (chemical shifts d in ppm and coupling
constants in Hz)
Results and discussion
Synthesis and characterisation of phosphino disilenes
29
29
31
Compound d Sia d Sib
1J(Si,Si)
d P
1J(Si,P) 2J(Si,P)
Reaction of one equivalent of the appropriate P-chlorophosphines
with disilenide 1 in toluene at room temperature afforded the
yellow to red-orange phosphino disilenes 2a-d (R = Ph (2a),
a
a
a
2a
2b
2c
2d
3
5a
5b
54.4
53.2
52.5
53.0
51.7
-3.3
1.1
96.5
96.6
95.4
98.8
76.8
43.9
48.0
-45.7
-21.9
-36.3
8.9
—
-33.2
-21.5
116
118
ca. 115
133
—
105
105
76
ca. 60
ca. 60
88
—
29
t
iPr (2b), Cy (2c), Bu (2d)) in 95 to 99% purity without further
143
purification (Scheme 1). Despite the high solubility of 2a-d in all
inert organic solvents, 2a and 2c could be crystallised at 0 ◦C
from a minimum amount of hexane, which allowed for obtaining
meaningful UV/vis data and combustion analysis in these cases.
128
a
90
31
a Not observed.
=
the Si Si bond. The value of 143 Hz is smaller than the one
1
=
observed for tetraaryl disilenes, e.g. Tip2Si Si(Tip)Ph ( J(Si,Si) =
151.4 Hz)16 suggesting a larger s-character of the bonding. This
observation coincides with a relatively large P,Si coupling constant
(Table 2). It may therefore be attributed to a decreased Si–Si double
bond character due allylic delocalisation brought about by the
planarising effect of the two t-butyl groups at the phosphorus
atom.
Scheme 1 Synthesis of phosphino disilenes 2a-d (Tip = 2,4,6-triisopropy-
i
t
lphenyl, 2a: R = Ph, 2b: R = Pr, 2c: R = Cy = cyclohexyl, 2d: R = Bu).
All phosphino disilenes 2a-d show distinct set of two 29Si NMR
doublets due to the coupling with the 31P nucleus (Table 2). The
signals of 2b-c are considerably broadened, presumably due to
hindered rotation around the Si–P bond, which can be expected
to lead to the presence of two or more rotamers. The 29Si chemical
shifts, ranging from 52.5 to 54.4 ppm for the phosphorus-bound
silicon atom and from 95.4 to 98.8 ppm for the b-Si atom, are fairly
independent of the substituents on phosphorus and of the same
magnitude as the previously reported monosilyl disilenes.12d,13 The
excellent signal-to-noise ration in case of the 29Si NMR spectrum
of the t-butyl derivative 2d allowed for the observation of 29Si
As to be expected, the 31P NMR shifts (Table 2) are much
more variable, but follow the well known trend associated to the
differing steric (and electronic) demand of the groups attached to
phosphorus as described by the Tolman cone angle.17 The con-
stitution of compounds was deduced from the observed coupling
patterns in a straightforward manner (though the couplings were
partially obscured by the broad signals in case of 2b and 2c). Not
surprisingly, the J(Si,P) increases with the steric demand of the
substituents at phosphorus, because the s-character of the Si–P
bond is increased as an sp2-like situation is approached. This is
1
1
satellites and thus the determination of the J(Si,Si) coupling of
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9288–9295 | 9291
©

particularly obvious in case of the t-butyl derivative 2d with its
relatively small Si,Si coupling constant (vide supra).
of 1, we reacted disilenide 1 with a very slight excess (1.02
eq) of iodine at -80 ◦C affording orange iodo disilene 3 in
spectroscopically almost quantitative yield (Scheme 2).
UV/vis spectroscopy of 2a and 2c in hexane solution (the
two phosphino disilenes of reliable purity, because obtained in
crystalline form) revealed longest wavelength absorptions and
*
extinction coefficients typical of the p–p transition of triaryl
disilenes12,13,16 (lmax = 423 (2a), 416 (2c) nm; e = 18400 (2a), 19000
(2c) M-1cm-1).
X-ray diffraction on single crystals of 2c confirmed the struc-
tural assignment based on the spectroscopic data (Table 1, Fig. 1).
Scheme 2 Synthesis of iodo disilene 3 and alternate generation of
diphenylphosphino disilene 2a by an “umpolung” strategy (Tip =
2,4,6-triisopropylphenyl).
˚
The Si(1)–Si(2) bond distance with 2.1542(11) A is at the short
=
end of what has been observed for Si Si bonds. The P–Si distance
The isolation of 3 from the reaction mixture, however, proved
tedious due to the apparent thermal instability of 3. In solution is
decomposes to unidentified brownish products within days even
at -30 ◦C. Nonetheless, we were able to isolate 3 as yellow crystals
in 12% yield by crystallisation from hexane. In crystalline form,
the orange 3 is persistent at -30 ◦C.
˚
(P(1)–Si(1) 2.2367(12) A) is relatively short as well considering
the bulk of the substituents. For comparison, the P–Si distance in
HC(SiMe3)2(SiMe2PPh2) – arguably with less bulky substituents
18
˚
– has been determined to 2.295 A. The short P–Si bond in 2c
is presumably due to the smaller coordination number of Si1,
but may in fact also indicate a certain allylic delocalisation in
accordance with the NMR spectroscopic findings for the t-butyl
derivative 2d. The sums of angles around both silicon atoms are
The 29Si NMR signals of 3 at 76.8 and 51.7 ppm are assigned
to the Tip2Si and TipSiI fragments, respectively, on the basis of
2D 29Si/1H correlations. The good signal to noise ratio again
allowed for the observation of 29Si satellite signals and thus the
determination of the one-bond coupling constant (1J(Si, Si) =
128 Hz). The significantly smaller value in comparison to those
of phosphino disilene 2d and other triaryl disilenes12,13,16 seems
to be at odds with Bent’s rule that s-character of orbitals tends
to be directed towards the more electropositive groups. On the
other hand, according to the CGMT model20 electronegative
close to the 360◦ of a trigonal planar coordination environment
ꢀ
(
angles = 359.47◦ (Si(1)), 358.99◦ (Si(2)). Distortions from
planarity are small: the trans-bent angles q (derived from the angle
between the Si(1)–Si(2) vector and the normal of the planes defined
by the silicon atom and the pending atoms) are moderate (qSi(1)
=
7.7◦, qSi(2) = 8.9◦), as is the twisting between the two planes (t =
3.5(1)◦).
=
substituents lead to increased trans-bending of the Si Si bond,
and therefore decrease the direct overlap between s-type orbitals.
The structure of 3 in the solid state was determined to address
˚
this question (Fig. 2). The Si(1)–Si(2) distance of 2.1914(9) A is
=
the first indication of a weaker Si Si bond in accordance with the
CGMT model. In line with that observation, the trans-bent angles
are very large (qSi(1) = 25.7◦, qSi(2) = 30.6◦). For comparison, the
=
trans-bent angles of Tip2Si Si(Tip)Ph were determined to qSi(1)
=
◦
23.6◦ and qSi(2) = 22.3 . The Si(1)–I distance of 2.4520(7) A is of a
˚
typical length.
Fig. 1 Structure of 2c in the solid state. Ellipsoids drawn at 50%
probability. H atoms and disordered cyclohexyl group omitted for clarity.
◦
˚
Selected bond lengths [A] and angles [ ]: Si(1)–Si(2) 2.1542(11); Si(1)–P(1)
2.2367(12); Si(1)–C(1) 1.898(3); Si(2)–C(16) 1.904(3); Si(2)–C(31) 1.892(3);
Si(2)–Si(1)–P(1) 110.51(5); C(52)–P(1)–C(46) 102.6(2); C(46)–P(1)–Si(1)
107.39(12); C(52)–P(1)–Si(1) 108.2(2).
Synthesis, characterisation and reactivity towards Ph2PLi of an
iodo disilene
Even though the syntheses of phosphino disilenes 2a-d worked
well after some optimisation, initial problems with unwanted side
reactions prompted us to explore the possibility of an “umpolung”
strategy. Recently, Tokitoh et al. reported the reaction of a
1,2-dibromo disilene with organyl lithium to afford the mono
substitution products.19 In order to obtain an electrophilic version
Fig. 2 Structure of 3 in the solid state. Thermal ellipsoids drawn at 50%
probability. H atoms omitted for clarity. Selected bond lengths [A] and
˚
angles[◦]:Si(1)–Si(2) 2.1914(9); Si(1)–I(1) 2.4520(7); Si(1)–C(1) 1.874(2);
Si(2)–C(16) 1.902(2); Si(2)–C(31) 1.898(2); Si(2)–Si(1)–I(1) 113.86(3);
C(1)–Si(1)–I(1) 110.00(7); C(1)–Si(1)–Si(2) 128.20(7); C(31)–Si(2)–C(16)
108.96(9); C(31)–Si(2)–Si(1) 115.81(7); C(16)–Si(2)–Si(1) 124.07(8).
9292 | Dalton Trans., 2010, 39, 9288–9295
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The Royal Society of Chemistry 2010
©

Addition of Ph2PLi (prepared by deprotonation of Ph2PH with
n-butyl lithium) to one equivalent of 3 at -90 ◦C affords 2a as the
only phosphorus containing product according to the 31P NMR.
The 1H NMR spectrum contained all characteristic signals of 2a.
However, a significant (apparently phosphorus-free) impurity was
detected by 29Si NMR with signals at -41.4 and -69.1 ppm, which
was also observed in the product mixture of the synthesis of the
starting material 3 and was therefore possibly carried through from
this stage of the synthesis.
Nonetheless, the “umpolung” strategy appears to be a feasible
=
alternative for the introduction of the Tip2Si SiTip-moiety. Given
the considerable additional effort, however, this reaction sequence
would only be a sensible option if either the nucleophilic functional
disilene or the electrophilic substrate is unavailable, hazardous or
difficult to access.
Fig. 3 Structure of cis-4 in the solid state. Thermal ellipsoids drawn at
50% probability. H atoms except for H1 and isopropyl groups omitted
◦
˚
for clarity. Selected bond lengths [A] and angles[ ]:Si(1)–Si2 2.3601(9);
Si(2)–P(1) 2.3075(9); Si(1)–H(1) 1.37(2); Si(1)–C(1) 1.904(2); Si(1)–C(47)
1.889(2); Si(2)–C(16) 1.917(2); Si(2)–C(31) 1.913(2); P(1)–C(46) 1.844(3);
P(1)–C(52) 1.849(3); C(47)–Si(1)–Si(2) 97.59(8); P(1)–Si(2)–Si(1) 90.57(3);
C(46)–P(1)–Si(2) 97.97(8).
Thermal rearrangement of phosphino disilenes 2a,c
In the case of a chlorodiphenylsilyl disilene,12d we had observed
the formation of a trisilaindane derivative upon melting, which
was explained by a transient silylene insertion into one of the
ortho–CH bonds of one of the phenyl groups. Melting of neat 2a
should produce the 1-phospha-2,3-disilaindane in an analogous
fashion, invoking a 1,2-phosphino shift to produce a transient
silylene intermediate (Scheme 3).21
Si(1)–Si(2) 97.59(8)◦; P(1)–Si(2)–Si(1) 90.57(3)◦). This finding is in
line with the usual reluctance of third row elements to hybridise,
but only possible due to a strong envelope distortion of the five-
membered ring (dihedral angles C(46)–P(1)–Si(2)–Si(1) 29.19(8),
C(47)–Si(1)–Si(2)–P(1) 28.92(8)◦).
In turn, this distortion brings the phenyl group close together
with the cis-substituent on the b-silicon atom; a situation, which is
much more favourable if this substituent is hydrogen (cis-4) rather
than a Tip group as in trans-4. Conversely, although we were
unable to obtain crystals of trans-4 the increased 1,3-interactions
would presumably flatten the five-membered ring, which would
lead to larger endocyclic angles and thus more s-character and
larger coupling constants.
The cyclohexyl substituted 2c similarly undergoes a CH-
insertion reaction upon heating, presumably yielding a mixture
of two diastereomers as well as suggested by two sets of high-field
29Si and 31P NMR signals very similar to that of 4. Due to non-
crystalline nature of the products, however, the constitution of the
insertion products remains to be confirmed.
Scheme 3 Thermal rearrangement of 2a to a mixture of diastereomers of
CH insertion product 4 (Tip = 2,4,6-triisopropylphenyl).
Thus, keeping a solid sample of diphenylphosphino disilene
2a at 140 ◦C for one hour affords a mixture of diastereomers in a
65 : 35 ratio. Crystallisation from hexane only allows for a marginal
enrichment of one of the isomers (72 : 28) of 4.
1
On grounds of their two sets of 29Si satellites, the H NMR
signals at 6.01 (72%) and 5.64 (28%) are identified as due to Si–H
groups, tentatively assigned of cis- and trans-4, respectively. Both
signals exhibit virtually identical coupling constants to two 29Si
nuclei (cis-4: 1J(Si,H) = 192 Hz, 2J = 39 Hz; trans-4: 1J(Si,H) =
Complexation of phosphino disilenes 2a,b to a 12-electron
palladium fragment
The Pd(0) precursor [Pd(PCy3)2] has previously been used by
Kira and co-workers for the synthesis of disilene complexes with
a p-complex-like coordination mode of the Si–Si double bond.9
This encouraged us to apply the same reagent towards phosphino
disilenes 2a and 2b (Scheme 4).
2
191 Hz, J = 40 Hz). Due to the fact that meaningful NOESY
spectra could not be obtained, the assignment of the more intense
signal to the cis-diastereomer is based on the assumption of lower
steric strain in cis-4 due to reduced 1,3-diaxial interactions. The
29Si and 31P NMR spectra of the diastereomeric mixture reveal
interesting differences in the coupling constants. The 1J(Si,P) and
2J(Si,P) coupling constants are significantly smaller in the major
isomer, cis-4, than in trans-4, suggesting a larger p-character in
the P–Si bond (cis-4: 1J (Si,P) = 35 Hz, 2J(Si,P) = 2 Hz; trans-4:
1J(Si,P) = 49 Hz, 2J(Si,P) = 20 Hz).
The structure of cis-4 determined by X-ray diffraction on single
crystals reveals a possible explanation for this observation (Fig. 3).
The endocyclic angles at P(1), Si(1), and Si(2) are considerably
smaller than the angles in an ideal pentagon (108◦) and thus show
a relatively high p-character (C(46)–P(1)–Si(2) 97.97(8)◦, C(47)–
Scheme 4 Formation of phosphine disilene complexes 5a,b (Tip =
i
2,4,6-triisopropylphenyl, Cy = cyclohexyl, 2a: R = Ph, 2b: R = Pr).
Indeed, the reactions of 2a and 2b with [Pd(PCy3)2] pro-
ceed smoothly at 25 and 50 ◦C, respectively, with concomitant
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9288–9295 | 9293
©

elimination of free PCy3. The disilene complexes 5a,b are formed
in almost quantitative spectroscopic yield. Isolation of pure 5a,b,
however, proved to be tedious and only 5a could be crystallised in
approx. 10% yield forming crystals suitable for X-ray diffraction.
The structure of the 5a,b can be readily derived from the
spectroscopic data. In particular, the 29Si NMR is instructive with
the two doublets of doublets, each signal being high-field shifted by
approximately 50 ppm with respect to the free phosphino disilene
=
ligand (Fig. 4, Table 2). This implies a coordination of the Si Si
bond to the palladium centre. The short relaxation times T1 of
the 29Si nuclei of 2b (T1 ª 1.0 s for d = 48.0 ppm; T1 ª 1.5 s
for d = 1.1 ppm) allowed for the sufficiently rapid accumulation
of the 29Si NMR spectrum and therefore the observation of the
29Si satellites for 2b. The thus determined coupling constant of
1J(Si, Si) = 90 Hz is distinctly smaller than for the free ligand:
*
=
suggesting considerable backbonding from Pd to the Si Si p
orbital. The 29Si–31P couplings constants are significantly smaller
as well, which indicates a lower s-character of the P–Si bond
Fig. 5 Structure of 5a in the solid state. Thermal ellipsoids drawn at
=
implying that the donation from the Si Si bond to the palladium
50% probability. H atoms and isopropyl groups as well as co-crystallised
centre is predominantly effected by s-type orbitals. The 31P NMR
signals of the phophino groups of the disilenes in the complex
5a,b show very similar chemical shifts to that of 2a,b providing
a first indication that the phosphorus atom does not directly
coordinate to the metal in either complex. The small 31P–31P
coupling constant of 3 Hz in 5a (not observed in 5b) supports
◦
˚
hexane omitted for clarity. Selected bond lengths [A] and angles[ ]:Pd–Si(1)
2.3764(4); Pd–Si(2) 2.3424(4); Pd–P(2) 2.4604(4); Si(1)–Si(2) 2.2569(5);
Si(1)–P(1) 2.2696(5); Si(1)–C(1) 1.9145(14); Si(2)–C(16) 1.9027(14);
Si(2)–C(31) 1.9194(14); P(1)–C(46) 1.8528(16); P(1)–C(52) 1.8404(15);
P(2)–C(58) 1.8561(17); P(2)–C(64) 1.8665(15); P(2)–C(70) 1.8577(16);
Si(2)–Pd–Si(1) 57.142(12); Si(1)–Pd–P(2) 156.049(13); Si(2)–Pd–P(2)
140.325(13).
3
2
this interpretation by being more in line with a J than with a J
=
coupling. Apparently, the strong s-donation by the Si Si bond as
well as the considerable p-backbonding from the Pd centre easily
compensates the undoubtedly much more pronounced steric strain
imposed by the observed coordination mode, which essentially
reflects the low thermodynamic stability of Si–Si p-bonding.
˚
2.3424(4) A), the pending phosphino group does not participate in
the coordination sphere of the transition metal. The coordination
environment is similar to that of the disilene p-complexes 6
and 7.9
The much more significant steric congestion in 5a in concert
=
with the unsymmetrical substitution pattern at the Si Si bond,
however, leads to a few differences with respect to 6. The
two Si–Pd–P angles (Si(1)–Pd–P(2) 156.049(13)◦; Si(2)–Pd–P(2)
140.325(13)◦) are much closer to one another than what had been
observed by Kira for 6 (128.94 and 171.83◦).9
In addition, while in 6 the P atom of the phosphine ligand lies
approximately in the Si–Si–Pd plane, in 5a the deviation of P(2)
from the Si(1)–Si(2)–Pd plane amounts to 0.5953(5) A. A similar
Fig. 4 29Si NMR signals of 5b showing the doublet of doublet coupling
patterns and 29Si satellites.
˚
deviation has been found for 1,2-disilacyclohexene complex 7,
=
The UV/vis spectrum of 5a shows two considerably overlapping
transitions at lmax = 491 nm and 443 nm. The extinction
coefficients at maximum absorption are both of the same order of
magnitude with approximately e = 4000 M-1cm-1, which is less than
a quarter of that of the precursor 2a. The very broad appearance
of the bands prevents further interpretations, but a charge transfer
contribution involving the Pd centre can be considered as likely.
The solid state structure of 5a confirms the structural features
in which the [Z]-geometry of the Si Si bond accounts for the
necessary discrimination in terms of steric demand. The Si(1)–
Si(2) bond of 5a (compared to 2c due to the absence of solid-
state structural information of 2a) is elongated from 2.1542(11)
˚
to 2.2569(5) A, hence by 4.8% (6: 3.2%; 7: 1.9%). The bent-back
angles q (5a: qSi1 = 16.0◦, qSi2 = 17.2◦, which are another criterion
for the degree of p-back-donation, are considerably larger than
in 6 (qSi1 = 4.4◦, qSi2 = 9.7◦) and 7 (qSi1 = 6.7◦, qSi2 = 7.2◦). The
sum of observation provides unambiguous prove for a substantial
metallacyclopropane character of 5a,b.
=
deduced from the spectroscopic data (Fig. 5). While the Si Si
moiety coordinates to the Pd centre (Pd–Si(1) 2.3764(4), Pd–Si(2)
9294 | Dalton Trans., 2010, 39, 9288–9295
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The Royal Society of Chemistry 2010
©

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(b) E. Pham and R. West, J. Am. Chem. Soc., 1989, 111, 7667; E. Pham
and R. West, J. Am. Chem. Soc., 1996, 118, 7871; (c) D. H. Berry, J. H.
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Conclusions
We reported two different synthetic access routes to phosphino-
substituted disilenes of type 2: the first makes use of the established
nucleophilic reagent, disilenide 1; the other route employs the
electrophilic iodo disilene 3, which is the first example of an
iodo functionalised disilene. In terms of thermal stability, phos-
phino disilenes resemble the corresponding chlorosilyl disilenes,
previously reported by us. In close analogy to these chlorosilyl
disilenes, the phosphino disilenes 2a,c rearrange into 1-phospha-
2,3-disilaindanes upon melting, presumably via a transient silylene
derivative.
Most importantly, the first transition metal complexes of silicon
analogues to vinyl phosphines have established the stark contrast
=
that the Si Si double bond coordinates in preference to the P-
Atom of the phosphino group benefitting from considerable p-
backdonation despite the coordination to a very electron-poor
12-electron [PdPCy3] fragment.
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Acknowledgements
Parts of the experiments were carried out while still at the
Institute of Inorganic Chemistry at the University of Wu¨rzburg.
Dr Ru¨diger Bertermann (Wu¨rzburg) is thanked for recording
NMR spectra there. The financial support of the Deutsche
Forschungsgemeinschaft (DFG SCHE 906/3-1 and 3-2) and the
Karl Winnacker-Funds of the Aventis Foundation is gratefully
acknowledged.
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