Isolation of a bis(silyl)palladium complex and Si–Si bond formation on a mononuclear palladium complex

Article abstract of DOI:10.1080/00958972.2018.1483076

Treatment of a chelating silane 1-(dimethylsilyl)-2-silylbenzene (4) with Pd(PEt₃)₄ in the ratio of 2:1 selectively afforded a unique mononuclear palladium complex [{1,2-C₆H₄(SiMe₂)(SiH)}₂Pd(PEt₃)₂] (6), which was formed by two Si–Si coupling reactions between the SiH₂ groups and the SiMe₂ moieties from two hydrosilane ligands. Complex 6 was structurally characterized by single crystal X-ray analysis and multinuclear NMR spectroscopic studies and is one of the very few examples of cis-bis(silyl)palladium(II) complexes containing in situ formed Si–Si single bonds.

Full text of DOI:10.1080/00958972.2018.1483076

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Isolation of a bis(silyl)palladium complex and Si-
Si bond formation on a mononuclear palladium
complex
Yan Rong Su, Jia Wei Liu, Fei Fei Ju, Jing Wen Xu, Qing Lu, Kang Xiao, Shi Wang
&
Yong Hua Li
To cite this article: Yan Rong Su, Jia Wei Liu, Fei Fei Ju, Jing Wen Xu, Qing Lu, Kang Xiao,
Shi Wang & Yong Hua Li (2018): Isolation of a bis(silyl)palladium complex and Si-Si bond
formation on a mononuclear palladium complex, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2018.1483076
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Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2018.1483076
Isolation of a bis(silyl)palladium complex and Si-Si bond formation on a
mononuclear palladium complex
YAN RONG SU, JIA WEI LIU, FEI FEI JU, JING WEN XU, QING LU, KANG XIAO*, SHI WANG and
YONG HUA LI*
Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors,
Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
(SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China
Treatment of a chelating silane 1-(dimethylsilyl)-2-silylbenzene (4) with Pd(PEt ) in the ratio of
3
4
2
:1 selectively afforded a unique mononuclear palladium complex [{1,2-
C H (SiMe )(SiH)} Pd(PEt ) ] (6), which was formed by two Si-Si coupling reactions between
6
4
2
2
3 2
the SiH groups and the SiMe moieties from two hydrosilane ligands. Complex 6 was
2
2
structurally characterized by single crystal X-ray analysis and multinuclear NMR spectroscopic
studies and is one of the very few examples of cis-bis(silyl)palladium(II) complexes containing
in situ formed Si-Si single bonds.
Keywords: Silyl; Palladium; In situ; Coupling; Multinuclear NMR
1
. Introduction
Silyl transition metal complexes have been investigated by many research groups because of
their unique bonding motifs and chemical properties. Oxidative addition of different types of
primary, secondary, and tertiary hydrosilanes with transition-metal species is the most effective
method for the generation of silyl complexes containing silicon-metal bonds. A number of novel
*
Corresponding authors. Email: iamyhli@njupt.edu.cn (Y.H. Li); iamkxiao@njupt.edu.cn (K. Xiao)
1

complexes including mononuclear or multinuclear structures with different valency have been
elucidated in the past few decades [1-11].
Among these transition metal silyl complexes, silyl palladium complexes have been
proposed as versatile intermediates in a number of palladium-catalyzed-transformations of
organosilanes, including the dehydrogenative Si-Si coupling reactions, hydrosilylations, and bis-
silylations despite the relatively poor stability compared with analogous platinum complexes
[12]. Since an understanding of the basic reactivity of palladium silyl compounds is important to
clarify their roles as catalytic intermediates, the preparation of silyl palladium complexes is of
great significance. Although a number of structurally characterized silylmetal complexes for
palladium have been described so far, silyl palladium complexes containing Si-Si single bonds
formed in situ are very limited. These rare mononuclear or multinuclear species with Si-Si single
bonds are considered to have potential application in the field of catalytic processes and
optoelectronic functional materials [13-16].
We have been studying stoichiometric reactions of various types of arylsilanes including
chelating, linear and/or branched structures with transition metal substrates, and reported a
variety of novel structures depending on the metals, the structures of the ligands, and the silyl
hydrides [13, 15, 17]. It has been a major synthetic goal and challenging work for many
researchers to isolate stable palladium silyl species in recent years. Because of this surge of
interest, our present research focused on investigations concerning the preparation of stable silyl
palladium complexes with novel structures and chemical reactivities.
The chelating silyl hydride 1,2-disilylbenzene (1) that reacted with Me Pd(dmpe) (dmpe
2
=
1,2-bis(dimethylphosphino)ethane) can lead to the palladium(IV) complex 3 with tetrakis(silyl)
ligands through a bis(silyl)palladium(II) intermediate, complex 2 (scheme 1) [8]. Herein,
replacing 1,2-disilylbenzene (1) with 1-(dimethylsilyl)-2-silylbenzene (4) afforded a very
different structure, 6, which was selectively prepared by the reaction of 4 with a palladium(0)
complex bearing Et P ligands.
3
2

2
2
. Experimental
.1. Materials and measurement
The experimental procedures were carried out under a nitrogen atmosphere in a glove box or
using Schlenk lines. Silyl hydride 4 was synthesized using a slightly modified method according
to our previous report [17b]. Extending the reaction time and increasing the amount of lithium
aluminium hydride can lead to a relatively higher yield. The characterization data of the product
were almost identical to those in the literature. Pd(PEt ) was prepared according to the
3
4
1
31
29
corresponding reference [18]. NMR spectra for H, P and Si were recorded on a Jeol LA500
for solution NMR). Chemical shifts are given in ppm and coupling constants are reported in
(
Hertz. Elemental analyses for C, H and N were performed on a Perkin-Elmer 240C elemental
analyzer (USA). All reagents employed in the present work were of reagent grade as received
from commercial sources (Aldrich). All solvents except deuterosolvents were distilled from
Na/benzophenone ketyl and stored over molecular sieves (4 Å).
2
2
.2. Synthesis
.2.1. Preparation of 1-(dimethylsilyl)-2-silylbenzene (4). A solution of 1-
dimethyl(isopropoxy)silyl-2-(triisopropoxysilyl)benzene (17 g, 0.043 mol) in ether (60 mL) was
added dropwise to an ether suspension (80 mL) of LiAlH (8 g, 0.21 mol) at 0 C over 30 min.
4
The resulting mixture was stirred for 4 h at room temperature followed by 12 h of reflux. Then,
additional LiAlH (1.0 g, 0.026 mol) was added and the mixture was refluxed for another 15 h.
4
After removal of volatile substances under reduced pressure, the remaining mixture was
extracted with hexane (150 mL × 3), and then filtered. Hexane was removed and the product
was transferred to a flask under high vacuum. Purification by distillation gave 1-(dimethylsilyl)-
1
2
-silyl-benzene, 134 C/50 Torr, as a colorless liquid, 4.8 g (0.029 mol), yield: 67%. H NMR
(
CDCl , 300 MHz)  0.39 (6H, d), 4.31 (3H, s), 4.65 (1H, septet), 7.34-7.44 (1H, m), 7.33-7.46
3
1
3
(
1H, m), 7.58 (1H, dd), 7.69 (1H, dd). C NMR (CDCl 75 MHz)  -3.26, 128.58, 129.28,
3
,
1
34.23, 135.41, 137.52, 138.23.
3

2
.2.2. Preparation of bis[benzene-1-(dimethylsilyl)-2-silyl]bis(triethyl phosphine)palladium
(
6). To a toluene solution (2 mL) of Pd(PEt ) (116 mg, 0.20 mmol) was added, dropwise, a
3
4
solution of 1-(dimethylsilyl)-2-silyl-benzene (4, 67 mg, 0.40 mmol) in toluene (2 mL) at 0 C
over 20 min. The resulting mixture was stirred first at room temperature for 12 h and then at
5
0 °C for 24 h to give a light yellow suspension. Removal of volatile substances under reduced
pressure gave a light yellow residue, which was washed with toluene-hexane (1:2, 2 mL × 4) and
dried under vacuum to afford the product 6 as a colorless solid, 55 mg (82.31 mmol), yield: 41%.
3
1
1
2
1
P{ H} NMR ([D ]THF, 202.0 MHz): for 6, δ 9.85 (s, J = 60 Hz). H NMR ([D ]THF,
8
Si-P
8
4
0
7
99.1 MHz): for 6, δ 0.12 (s, 6H, SiMe ), 0.16 (s, 6H, SiMe ), 1.86-1.97 (m, 12H, (-CH -)×6),
2 2 2
.88-1.01 (m, 18H, (-CH )×6), 4.02 (t, 2H, J = 26 Hz, SiH), 7.07-7.13 (m, 4H, aromatic-H),
3
H-P
2
9
1
.35 (d, 2H, JH-H = 6 Hz, aromatic-H), 7.75 (d, 2H, JH-H = 6 Hz, aromatic-H). Si{ H} NMR
(
[D ]THF, 99.1 MHz, DEPT, the measurement parameters were adjusted to the silicon atoms
8
with directly bound hydrogen atoms): for 6, δ -40.04 (t, 2JP-Si = 60 Hz, SiH). Si{ H} NMR
29
1
(
[D ]THF, 99.1 MHz, INEPT, the measurement parameters were adjusted to the silicon atoms
8
with two methyl groups): for 6, δ -16.76 (t, 3JP-Si = 2 Hz, SiMe ). Anal. Calcd. for C H P PdSi
4
2
28 52
2
(%): C, 50.24; H, 7.83. Found: C, 50.56; H, 7.58.
2
.3. X-ray crystallography
X-ray diffraction data were collected on a Bruker Smart Apex CCD area detector diffractometer
using Mo K (0.71073 Å) radiation. Cell parameters were retrieved using SMART software and
α
refined using SAINT [19]. Absorption corrections were applied using SADABS [20]. Structures
were solved by direct methods using SHELX-97 [21] program package. The hydrogen atoms
were placed in idealized positions using a riding model. Compound 6 was refined to convergence
using anisotropic thermal parameters for all non-hydrogen atoms.
4

3
. Results and discussion
A stable bis(silyl) palladium complex was prepared from the direct reaction of a chelating silyl
hydride precursor with a Pd(0) complex. Disilane ligand 4 involves a characteristic trihydrosilyl
group, which leads to its high reactivity toward transition metals. Reaction of 1-(dimethylsilyl)-
2
-silylbenzene 4, which did not contain Si-Si single bonds, with Pd(PEt ) in a 2:1 ratio in
3 4
toluene afforded the cis-bis(silyl) palladium complex, [{1,2-C H (SiMe )(SiH)} Pd(PEt ) ] (6)
6
4
2
2
3 2
exclusively (scheme 2). The whole reaction process underwent a sequence of oxidative addition
of the Si–H bond to the Pd center and reductive elimination steps accompanying the dissociation
of PEt molecules. Single-crystal X-ray structural analysis revealed interesting structural
3
characteristics of 6 which has two new Si-Si bonds formed in situ. Although the mechanism for
the syntheses of 6 is not clear at the moment, the in situ generated Si-Si bonds may be explained
by intramolecular dehydrogenative Si-Si coupling reaction between the SiH groups and the
2
SiMe moieties from two hydrosilane ligands through an intermediate of
2
(hydrido)tris(silyl)palladium(IV) (5) (scheme 3). The reaction of the chelating silane 4 with
Pd(PEt ) in 1:1 molar ratio also afforded the mononuclear palladium complex 6 as the main
3
4
product. Although a number of silyl complexes are known for palladium, structurally
characterized silyl palladium complexes containing Si-Si single bonds are very rare. To the best
of our knowledge, only 6 complexes with a bis(silyl)palladium structure which contains at least
one Si-Si single bond formed in situ has been registered in the Cambridge Structural Database,
among them, only silyl palladium(II) compound 1 possesses two Si-Si single bonds [13].
Crystals of 6 suitable for single-crystal X-ray analysis were grown by cooling a mixed
solvent of THF and hexane to 0 C, and the molecular structure of 6 was confirmed
unambiguously by X-ray crystallography, as depicted in figure 1. Complex 6 crystallizes in the
triclinic space group P -1 (table 1). The palladium center has a distorted-square-planar
environment with Si1-Pd1-Si3 = 76.05(3), P1-Pd1-P2 = 105.81(2)°, P1-Pd1-Si3 = 161.49(2)°,
and P2-Pd1-Si1 = 161.74(2)° bond angles, which is completed by two Si atoms from two
hydrosilane ligands and two P atoms from two Et P ligands. The P1-Pd1-P2 angle is increased
3
5

greatly from the ideal value of 90 and deviates obviously from the range typically observed for
Pd complexes stabilized by organophosphorus ligands. The average bond lengths of Pd-Si
(
[
(
2.36 Å) and Pd-P (2.35 Å) are all in good agreement with the reported P PdSi complexes
2 2
8, 12b, 13]. The Si2-Si3 bond length (2.3477(11) Å) is almost equal to that of the Si1-Si4 bond
2.3449(11) Å). The average Si-Si bond length (2.35 Å) is within common ranges compared with
those of the similar silyl palladium complexes containing Si-Si single bonds [13, 15, 16]. There
exists some deviation from the P PdSi plane at the Pd atom, probably due to steric repulsion
2
2
between Et P ligands and the methyl groups on the silicon center. The Pd1-Si1 bond distance
3
(2.3545(10) Å) is very close to that of the Pd1-Si3 bond (2.3590(8) Å) (table 2).
3
1
1
The P{ H} NMR spectrum of 6 in [D ]THF is consistent with the structure of 6; the two
8
chemically equivalent phosphorus atoms give rise to a singlet with a relatively moderate 2JSi-P
value which is smaller than those observed in similar cis-bis(silyl)bis(phosphine)palladium(II)
2
9
1
complexes [13]. The Si{ H} NMR signals (two triplets) with a moderate (60 Hz) and a small
2 Hz) JP-Si value are assignable to the silicon nuclei of SiH and SiMe groups, respectively.
(
2
1
29
Signals for SiH groups in the H NMR and proton-decoupled Si NMR spectrum are also
consistent with the structure. The P-Si and P-H signals (two triplets) with moderate 2JP-Si and 3JP-
values in 6 indicate that the SiH group is in cis-position relative to one P atom and trans to the
H
other P atom. The 3JH-P value in the SiH proton signals of 6 is larger than those of the reported
analogous complexes having Pd–SiH bonds [13, 16]. Furthermore, the values of 2JSi-P(trans) and
3JH-P(trans) are very close to those of 2JSi-P(cis) and 3JH-P(cis) in the Si and H NMR spectrum, which
29
1
may be due to the rapid site exchange between the two PEt ligands and thus average coupling
3
observed at room temperature [14b].
4
. Conclusion
A rare and stable cis-bis(phosphine)bis(silyl) palladium(II) complex 6 with two in situ formed
Si-Si single bonds obtained by successive oxidative addition, reductive elimination reactions of a
chelating hydrosilane ligand has been established. We report the synthesis, structural and
6

preliminary mechanism studies of this new complex. The present work has opened a new avenue
for exploration of dehydrogenative Si-Si coupling reaction mediated by silylpalladium species.
The structural unit containing Si-Si single bond is often an essential structural component of
optoelectronic functional materials. Further application of this bis(silyl) transition-metal complex
are currently in progress.
Supplementary material
CCDC-1580101 contains the supplementary crystallographic data for 6. These data can be
obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was financially supported by the National Key Basic Research Program of China
(Grant no. 2014CB648300), the National Natural Science Foundation of China (Grant
no. 21771110), the Natural Science Foundation of Jiangsu Province (BK20141425), the Priority
Academic Program Development (PAPD) of Jiangsu Higher Education Institutions and the
Ministry of Education of China (YX03002).
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9

1
0

1
1

Table 1. Crystallographic data for 6.
C H P PdSi
4
Formula
Formula weight
Crystal system
Space group
a, Å
28 52
2
669.42
Triclinic
P -1
9.7454(13)
11.3520(15)
16.547(2)
80.108(2)
81.730(2)
71.434(2)
1701.7(4)
2
b, Å
c, Å
α, deg

, deg
γ, deg
V, Å3
Z
T/K
100(2)
Dcalcd, g cm-3
1.306
F(000)
704
µ, mm-1
0.796
Index ranges
Reflections collected
Unique reflections
Data / restraints / parameters
GOF (F2)
−12 ≤ h ≤ 12, −14 ≤ k≤ 14, −21 ≤ l ≤ 21
7772
7567
7772 / 0 / 334
1.222
R indexes [I>2σ(I)]
R1 = 0.0298
wR = 0.0706
2
R (all data)
R1 = 0.0311
wR = 0.0711
2
2
2 2
2 2 1/2
R1 = ∑||F |-|F ||/∑| F |; wR2 = [∑w(F - F ) /∑w(F ) ]
o
c
o
o
c
o
1
2

Table 2. Selected bond lengths (Å) and angles (°) for 6.
Pd1-Si1
2.3545(10)
2.3552(7)
2.3449(11)
1.899(2)
Pd1-Si3
2.3590(8)
2.3631(11)
2.3477(11)
1.904(2)
Pd1- P1
Pd1- P2
Si1- Si4
Si2- Si3
Si1-C1
Si3-C9
P2-C25
1.845(2)
P1-C19
1.843(2)
P1-Pd1-Si3
P1-Pd1-P2
P1-Pd1-Si1
Si2-Si3-Pd1
Pd1-P2-C25
Si1-Si4-C14
161.49(2)
105.81(2)
89.49(3)
P2-Pd1-Si1
Si1-Pd1-Si3
P2-Pd1-Si3
Si4-Si1-Pd1
Pd1-P1-C19
Si3-Si2-C6
161.74(2)
76.05(3)
90.64(3)
115.55(3)
118.48(7)
112.71(7)
114.59(4)
117.95(7)
112.92(7)
Graphical abstract
SiMe H
H
2
0oC to 50 C
o
Si
Si
PEt3
PEt3
Me Si
2
Pd(PEt3) + 2
4
Pd
-
2PEt3
Me Si
2
SiH3
H
-
H2
4
6
1
3