Isolation of two bis(silyl)nickel complexes with Si-Si bond formation in a single-crystal-to-single-crystal fashion

Article abstract of DOI:10.1039/c8dt04951c

A unique penta-coordinate nickel complex [{1,2-C₆H₄(SiMe₂)(SiH)}₂Ni(PEt₃)(depe)] (6) that generates two new Si-Si single bonds has been prepared exclusively by the reaction of a chelating silyl hydride 1-(dimethylsilyl)-2-silylbenzene (4) and Ni(depe)(PEt₃)₂ (depe = Et₂PCH₂CH₂PEt₂) in a 2:1 ratio. Complex 6 is the first example of mononuclear silyl nickel complexes containing a Si-Si single bond formed in situ. Interestingly, framework 6 exhibits reversible single-crystal-to-single-crystal transformations upon the removal and rebinding of the coordinating PEt₃ molecule, which give rise to a PEt₃-free complex [{1,2-C₆H₄(SiMe₂)(SiH)}₂Ni(depe)] (6′), concomitant with the alteration of the coordination geometry of central metal atoms.

Full text of DOI:10.1039/c8dt04951c

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Isolation of two Bis(silyl)nickel Complexes with Si–Si Bond
Formation in Single-Crystal-to-Single-Crystal Fashion
a
a
a
a
a
Sen-Yu Zhang • Jie Li • Sheng-Ze Zhao • Shi Wang *• Min-Ming Lu • Yong-Hua
a
ab
Li *• Wei Huang
a
Key Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for
Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications,
Nanjing 210023, China.
E-mail: iamswang@njupt.edu.cn; iamyhli@njupt.edu.cn
http://smse.njupt.edu.cn/
b
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
(
IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials
SICAM), Nanjing Tech University (NanjingTech), Nanjing 211816, China.
(
Keywords: Nickel; Silyl hydride; In situ; Si–Si single bond
A unique penta-coordinate nickel complex [{1,2-C H (SiMe )(SiH)} Ni(PEt )(depe)]
6
4
2
2
3
(6) that generates two new Si-Si single bonds has been prepared exclusively by the
reaction of a chelating silyl hydride 1-(dimethylsilyl)-2-silylbenzene (4) and
Ni(depe)(PEt ) (depe＝Et PCH CH PEt ) in 2:1 ratio. Complex 6 is the first example
3
2
2
2
2
2
of mononuclear silyl nickel complexes containing Si–Si single bond formed in situ.
Interestingly, framework 6 exhibits reversible single-crystal-to-single-crystal
transformations upon removal and rebinding of the coordinating PEt molecule, which
3
’
give rise to PEt free complex [{1, 2-C H (SiMe )(SiH)} Ni(depe)] (6 ), concomitant
3
6
4
2
2
with alteration of coordination geometry of central metal atom.
1

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Introduction
The research on the unique reactivity of versatile hydrosilanes toward
transition-metal complexes have attracted a broad array of attention since the initial
investigation of the first silyl metal complex. Oxidative addition of various silyl
hydrides with low-valent transition-metal species is one of the most important ways
for synthesizing silyl complexes including silicon-metal bonds. Many types of
complexes with novel structures and chemical properties have been well documented
via the method of Si−H bond activation during the past few decades.[1-9]
Among these silyl transition-metal complexes, silyl nickel complexes have seldom
been studied which may be due to the poor stability compared with those of platinum
complexes. Silyl nickel complexes are often postulated as crucial intermediates in the
process of nickel-catalyzed transformations of silicon compounds. Catalytic
transformations mediated by these species involve dehydrogenative Si–Si coupling,
hydrosilylation, bis-silylation and silylcarbonylations reactions of unsaturated organic
substrates.[10] Since the fundamental knowledge of the reactivity of nickel silyl species
is essential to understand their roles as catalytic intermediates, the devise and
synthesis of silyl nickel complexes have very important theoretical and practical
significance.
We have been engaged in the research on the reactivity of aliphatic and aromatic
silanes possessing linear or chelating structures with low-valent platinum and
palladium precursors bearing different ligands, and revealed a wide variety of
corresponding silyl group 10 transition-metal complexes.[11] For researchers, isolating
relatively stable silyl nickel species has been a very meaningful and challenging task
so far. Because of this surge of interest, we focus our research on the preparation of
silyl nickel complexes with unique reactivities and bonding structures.
Although a few structurally characterized silylmetal complexes for nickel have
been reported in recent years, nickel compounds bearing (bis)silyl ligands are limited
due to their instability, particularly scarce are complexes that contain Si NiP
2
2
framework and/or Si–Si single bonds. These rare species with such structural features
have practical applications in the field of organic catalysis.[12]
2

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The reaction of chelating disilane 1, 2-disilylbenzene (1) with low-valent nickel
precursor of [Ni(dmpe) ] in 2:1 ratio can result in the tetrakis(silyl)nickel(IV)
2
[5]
complex 3 (Scheme 1). Herein, 1, 2-disilylbenzene (1) was replaced by the similar
silyl hydride 1-(dimethylsilyl)-2-silylbenzene (4) which involves characteristic
dimethylhydrosilyl and trihydrosilyl groups, and has high reactivity toward transition
metals. A very different structure with Si–Si single bonds and Si Ni(II)P framework,
2
3
6
can be obtained unprecedentedly by the direct reaction of 4 with a nickel(0)
complex bearing phosphine ligand.
Experimental Section
Materials and Measurement
All experiments were operated under a nitrogen atmosphere in a glove box or by
standard Schlenk techniques. Ether, hexane, toluene, and THF were distilled from
sodium benzophenone. Hydrosilane 4 was prepared using a slightly modified method
according to the previous report.[12d] Ni(PEt ) was synthesized according to literature
3
4
procedure.[13] 1, 2-bis(diethylphosphino)ethane (Aldrich) was purchased and used
1
31
29
without purification. H (499.1 MHz), P (202.0 MHz), and Si (99.1 MHz) NMR
spectra were recorded on Jeol LA500 (for solution NMR). IR spectra were recorded
on PE-Spectrum Two. Unless otherwise specified, the NMR spectra were recorded at
room temperature (ca. 25 °C), and the chemical shift values are reported in ppm (δ).
Coupling constants are reported in Hertz. The elemental analyses were carried out by
the Perkin-Elmer 240C elemental analyzer (USA).
Synthesis
Preparation of 1-(dimethylsilyl)-2-silylbenzene (4)
A solution of 1-dimethyl(isopropoxy)silyl-2-(triisopropoxysilyl)benzene (8.5 g, 0.022
mol) in ether (35 mL) was added dropwise to an ether suspension (40 mL) of LiAlH4
o
(
4 g, 0.11 mol) at 0 C during 30 min. 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
4
mol) was added and the mixture was refluxed for another 15 h. After removal of
3

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volatile substances under reduced pressure, the remaining mixture was extracted with
hexane (100 mL x 3), and then filtered. Hexane was removed and the product was
transferred to a flask under high vacuum. Purification by distillation gave 1-
0
(
dimethylsilyl)-2-silyl-benzene, 134 C/50 Torr, as a colorless liquid, 2.4 g (0.015
1
mol), yield: 67%. H NMR (CDCl 499.1 MHz)  0.39 (6H, d), 4.31 (3H, s), 4.65 (1H,
3
,
1
3
septet), 7.34-7.44 (1H, m), 7.33-7.46 (1H, m), 7.58 (1H, dd), 7.69 (1H, dd). C NMR
CDCl 75 MHz)  -3.26, 128.58, 129.28, 134.23, 135.41, 137.52, 138.23.
(
3
,
Preparation of {1,2-C H (SiMe )(SiH)} Ni(PEt )(depe) (6)
6
4
2
2
3
1
-(Dimethylsilyl)-2-silyl-benzene (4, 66 mg, 0.40 mmol) dissolved in 3 mL of toluene
was slowly added to a stirred solution of Ni(PEt ) (106 mg, 0.20 mmol) in 2 mL of
3
4
o
toluene at 0 C over 20 min. The low temperature was maintained for 10 min. Then
the resulting mixture was stirred at room temperature for 16h to give a light yellow
suspension. Removal of volatile substances under reduced pressure gave a light
yellow residue, which was washed with toluene-hexane (1:3, 2 mL × 4) and dried
under vacuum to afford the product 6 as a yellow-green solid, 88 mg (0.12 mmol),
yield: 62%. IR (KBr): ṽ = 3083, 3010, 2905, 1921, 1657, 1436, 1429, 1298, 1262,
138, 1069, 931, 879, 833, 811, 752, 724, 668, 605, 497, 462. 31P{ H} NMR
1
1
1
(
[D ]THF, 202.0 MHz): for 6, δ -9.33 (br s), 56.08 (br s). H NMR ([D ]THF, 499.1
8
8
MHz): for 6, δ 0.33 (s, 6H, SiMe ), 0.52 (s, 6H, SiMe ), 0.65-0.88 (m, 24H,
2
2
Et PCH CH PEt ), 1.22-1.75 (m, 15H, Et P), 4.54 (t, 2H, J
＝ 26 Hz, SiH × 2),
2
2
2
2
3
H-P
7
.23-7.27 (m, 4H, aromatic-H), 7.58 (d, 2H, JH-H ＝ 5 Hz, aromatic-H), 8.17 (d, 2H,
JH-H ＝ 5 Hz, aromatic-H). 29Si{ H} NMR ([D ]THF, 99.1 MHz, DEPT, the
1
8
measurement parameters were adjusted to the silicon atoms with directly bound
hydrogen atoms): for 6, δ -39.01 (br s, SiH), -39.78 (br, s, SiH). Anal. Calcd. for
C H NiP Si (%): C, 54.15; H, 8.66. Found: C, 54.54; H, 8.34.
3
2
61
3
4
Preparation of {1,2-C H (SiMe )(SiH)} Ni(depe) (6’)
6
4
2
2
Single crystals of 6 was evacuated at room temperature under vacuum for 10 h, which
almost quantitatively converted to cracked micro-crystalline solid, and then
recrystallized in THF under a nitrogen atmosphere to give green single crystals of 6’.
IR (KBr): ṽ = 3085, 3061, 2962, 2937, 1936, 1628, 1377, 1439, 1249, 1127, 1063,
4

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3
1
1
9
39, 835, 809, 749, 701, 659, 631, 452. P{ H} NMR ([D ]THF, 202.0 MHz): for 6’,
8
1
δ 60.49 (br s). H NMR ([D ]THF, 499.1 MHz): for 6’, δ 0.11 (s, 6H, SiMe ), 0.18 (s,
8
2
6
H, SiMe ), 0.87-0.93 (m, 6H, Et P), 0.96-1.02 (m, 6H, Et P), 1.78-1.91 (m, 8H,
2 2 2
(
P-Et) × 4), 1.98-2.11 (m, 4H, PCH CH P), 4.05 (t, 2H, J
＝ 23 Hz, SiH × 2),
2
2
H-P
7
.03-7.11 (m, 4H, aromatic-H), 7.35 (d, 2H, JH-H ＝ 7 Hz, aromatic-H), 7.76 (d, 2H,
JH-H ＝ 7 Hz, aromatic-H). 29Si{ H} NMR ([D ]THF, 99.1 MHz, DEPT, the
1
8
measurement parameters were adjusted to the silicon atoms with directly bound
2
9
1
hydrogen atoms): for 6, δ -39.37 (br s, SiH). Si{ H} NMR ([D ]THF, 99.1 MHz,
8
INEPT, the measurement parameters were adjusted to the silicon atoms with two
methyl groups): for 6, δ -16.14 (br s, SiMe ). Anal. Calcd. for C H NiP Si (%): C,
2
26 46
2
4
5
2.82; H, 7.79. Found: C, 52.51; H, 7.52.
X-ray crystallography
The X-ray structures were obtained using the sealing technology at room temperature,
and the data were collected using the SMART software on a Bruker APEX CCD
diffractometer using a graphite monochromator with Kα radiation (λ =0.71073 Å).
Data reductions were performed using the SAINT software, and the data were
corrected for absorption using SADABS.[14] The structures were solved by direct
2
methods and refined to convergence by full-matrix least squares on F with
anisotropic thermal parameters for all non-hydrogen atoms using SHELX-97 software
package.[15] The hydrogen atoms were placed in idealized positions using a riding
model.
Results and Discussion
Treatment of a chelating hydrosilane 1-(dimethylsilyl)-2-silylbenzene (4) with Ni(0)
complex of [Ni(PEt ) (depe)] (depe=1,2-bis(diethylphosphino)ethane) in 2:1 ratio in
3
2
toluene did not lead to the desired species of tetrakis(silyl)nickel(IV), dinuclear
µ-silylene)(silyl)nickel(III) or DEPE-bridged dimeric bis(silyl)nickel(II) as those of
(
the reaction for 1,2-disilylbenzene (1). A rare cis-bis(silyl)nickel compound
II
[
{1,2-C H (SiMe )(SiH)} Ni(PEt )(depe)] (6) with Si Ni P framework was
6
4
2
2
3
2
3
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generated unexpectedly (Scheme 2).
The entire reaction process include a sequence of oxidative addition of the Si–H
bond to the Ni center and reductive elimination steps accompanying the release of
dihydrogen and dissociation of triethylphosphine molecules, concomitant with
two-step Si–Si coupling reactions. The silyl hydride precursor 4 did not contain Si–Si
single bonds. However, Si–Si bond formation did take place during the reaction.
Single-crystal X-ray analysis revealed this interesting structural characteristics of
complex 6 which has two new Si-Si bonds. Although the mechanism for the
formation of complex 6 is not accurate at present, the Si–Si bonds may be envisioned
to be formed in situ by intramolecular Si–Si coupling reaction between the SiH2
groups and the SiMe moieties from two hydrosilane ligands (Scheme 3).
2
The reaction of the disilane 4 with [Ni(PEt ) (depe)] in 1:1 ratio also afforded the
3
2
bis(silyl)nickel complex 6 as the main product exclusively. Although some types of
silyl complexes are known for nickel, structurally characterized penta-coordinate
II
nickel complexes with Si Ni P framework structure are rarely reported, among them,
2
3
Si–Si single bond containing silyl nickel compounds are exceptionally limited.[12c-e, 12j]
To the best of our knowledge, complex 6 is the first example of mononuclear silyl
nickel complexes containing Si–Si single bond formed in situ.
Single crystals were obtained upon cooling down slowly the hot tetrahydrofuran
solution of 6 under a nitrogen atmosphere, and the solid-state structure of 6 was
elucidated by X-ray crystallography. Complex 6 crystallizes in the monoclinic space
group P 21/n (Table S1). Ni1 shows a distorted square pyramidal (sp) coordination
geometry, being coordinated with two Si atoms (Si1, Si3) of two hydrosilane ligands,
two P atoms (P1, P2) from DEPE ligand , as shown by the bond angles of P1-Ni1-Si3
(152.18(3)°), P1-Ni1-P2 (87.58 (3)°) and P2-Ni1-Si3 (90.43 (4)°) respectively (Figure
1
).[16] P3 from Et P ligand occupies the axial site of sp geometry. Et P is a relatively
3
3
II
weak ligand, there are few reports on Et P coordination to Ni ion bearing bis(silyl)
3
ligand.[12f-g] In 6, a Et P molecule coordinates a Ni ion with the Ni-P bond length of
II
3
2
.263(8) Å, which is a little longer than Ni-P (DEPE) bond lengths [av. 2.181(7) Å].
The average bond lengths of Ni-Si (2.27 Å) and Ni-P (2.21 Å) are almost consistent
6

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with those of reported Si NiP complexes. The bond length of Si1-Si4 bond (2.347 (1)
2
3
Å) is almost identical to that of the Si2-Si3 bond (2.374 (1) Å). The average Si–Si
bond length (2.36 Å) is within common ranges compared with those of the silyl
palladium complexes containing Si–Si single bonds.[11a-b, 12b] The dihedral angle
between the Si1Ni1Si3 and P1Ni1P2 plane is 41.55°. The central Ni deviates slightly
from the P2Si3Si1 plane, probably due to steric repulsion between ethyl and methyl
groups on the phosphorus and silicon atom (Table S3).
Interestingly, framework 6 exhibits reversible single-crystal-to-single-crystal
transformations upon removal and rebinding of the coordinating PEt molecule. The
3
Et P ligand of 6 can be liberated from this metal-organic framework easily by
3
evacuating at room temperature under vacuum for 10 h, and then recrystallization of
the resulting cracked micro-crystalline solid under a nitrogen atmosphere can afford
’
the green single-crystalline [{1,2-C H (SiMe )(SiH)} Ni(depe)] (6 ). This crystal
6
4
2
2
’
dynamics are reversible. When Et P-free single crystals of 6 are immersed in
3
triethylphosphine for 5 h at room temperature, the Et P ligand was recovered at the
3
metal center, which give rise to the original crystals of 6 (Scheme 4). The X-ray
structures show that host framework structure is retained during the transformations
from 6 to 6’ and from 6’ to 6. This phenomenon of reversible insertion and removal of
the organophosphorus ligand while preserving the single crystallinity is
unprecedented in silyl metal framework. Complex 6 is the first example of silyl nickel
complex with single-crystal-to-single-crystal transformation behavior.
’
Complex 6 crystallizes in the non-centrosymmetric tetragonal space group P -4c2
’
(
Table S2). The X-ray crystal structure of 6 reveals that the framework structure of 6
is retained, but the molecular rearrangements involving alteration of coordination
’
geometry of Ni1 occur in 6 (Figure 2). The coordination geometry of Ni1 changes
from a distorted square pyramidal to a distorted square planar with bond angles of
Si2-Ni1-Si2A (83.32 (3)°), P1-Ni1-P1A (90.19 (3)°) and P1-Ni1-Si2A (93.50 (2)°)
respectively. The dihedral angle between the Si2Ni1Si2A and P1Ni1P1A plane is
7
.86° (Table S4). Compared to 6, the average distance of Ni1-Si (2.24 Å) bond in 6’
decreased by 0.03 Å and the Ni1-P bond length decreased to 2.18 Å. On the other
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’
hand, the average Si–Si bond length (2.33 Å) in 6 decreased by 0.03Å. In the
’
transformation of 6 to 6 , the bond angles of Si1-Ni1-Si3 and P1-Ni1-P2 increase from
o
o
7
6.06(4) and 87.58(3) to 83.32 (3) and 90.19 (3) respectively. Very few structurally
II
[12f-i]
characterized nickel complexes involving Si Ni P framework have been reported.
2
2
Complex 6 is the first example of tetra-coordinate silyl nickel complexes containing
II
Si–Si single bond and Si Ni P framework structure.
2
2
’
Complex 6 and 6 were also structurally confirmed by multinuclear NMR
spectroscopic studies. The phosphorus spectrum signals of 6 at the high field and low
3
1
1
field are attributed to the P nuclei of Et P and DEPE ligand respectively.The P{ H}
3
’
’
NMR spectrum of 6 in [D ]THF is consistent with the structure of 6 , the two
8
2
9
1
chemically equivalent phosphorus atoms give rise to a singlet. The Si{ H} NMR
signals (broad singlets) are assignable to the silicon nuclei of SiH and SiMe groups,
2
1
respectively. The signals of the SiH groups (in the H NMR spectrum) are also
consistent with the structures. The P-H signals (triplet) with moderate 3JP-H values in
both compounds indicate that the SiH group is in cis-position relative to one P atom
and trans to the other P atom. The 3JH-P value in the SiH proton signals of compounds
6
and 6’ is larger than those of the reported analogous complexes bearing Ni–SiH
bonds and phosphine ligand.[17] Furthermore, the values of 3JH-P(trans) (26 Hz for 6, 23
Hz for 6’) is identical to those of 3JH-P(cis) in the H NMR spectrum, which may be due
to the rapid site exchange of the two phosphine atoms from DEPE ligand and thus
average coupling between hydrogen and phosphine nuclei observed at room
temperature.[18]
1
Concluding Remarks
In summary, we demonstrated isolation, structures and mechanism studies of two
rare tetra- and penta-coordinate nickel complexes bearing bis(silyl) and phosphine
ligands. The present result is the first report of a single-crystal-to-single-crystal
transformation in which an organophosphorus ligand is quantitatively and reversibly
binded to and removed from a silyl nickel(II) complex without changing the main
framework structure. These results also suggest a new route to construct the
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molecules containing Si–Si single bond mediated by silylnickel species, which is
often an essential structural component of optoelectronic functional materials. These
two coordinatively unsaturated Ni centres containing vacant coordination sites may
have potential chemical reactivity. Further investigation of these bis(silyl)
transition-metal complexes are currently in progress.
Acknowledgments
This work was financially supported by 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).
Supporting Information Available: The supporting information including additional
structural details, CIF files for 6 and 6’ is available free of charge via the Internet at
http://pubs.rsc.org.
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(
[
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2
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1
3

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H2
Si
Me2
P
SiH2
Ni
Me2
SiH3
SiH3
P
+ 1
H Si
2
+
Ni(dmpe)2
Ni
P
Me2
Si
H2
P
H Si
2
SiH2 Me
2
1
2
3
Scheme 1 Reaction of 1, 2-Disilylbenzene with Ni(dmpe)2
Et
P
PEt3
2
SiMe H
0oC to rt
H
2
Si
Si
Me2
Si
Ni(depe)(PEt3) + 2
2
Ni
-
PEt3
H2
Me Si
2
SiH3
P
Et2
H
-
4
6
Scheme 2 Synthesis of complex 6 containing two in situ formed Si–Si single bonds
1
4

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Me
Si
Me
Si
2
2
H
H Et2
Et2
P
SiMe H
H
H
2
P
H Si
H Si
2
OA, RE
2
RE
Ni(depe)(PEt3)
2 + 2
Ni
Ni
-
PEt3
SiH3
P
H Si
P
Et
H Si
2
2
-
H2
Et2
4
SiMe2
2
SiMe2
OA
5
Me
Me
Si
2
2
Si
SiMe2
H
H
Et
P
Et
P
2
Et
P
2
2
H
Ni
H
HSi
H Si
OA
HSi
RE
HSi
Ni
Ni
-H2
P
Et
P
P
Et
H
H Si
2
H Si
2
2
Et
2
2
SiMe2
SiMe2
SiMe2
2
RE
Et
P
Et
Et
P
2
2
2
H
H
H
H
P
H
Si
Si
Me2
Si
Si
OA
Me
Si
Si
Me2 Si
Me2Si
2 Si
RE
Ni
Ni
Ni
Me Si
Me
2Si
-H2
2
SiH2
P
P
Et2
P
Et2
H
H
H
Et
2
t 3
E
P
Et
P
PEt3
Ni
2
H
Me2
Si
Si
Si
Me Si
2
P
Et2
H
6
Scheme 3 Plausible Mechanism for the Formation of 6
Et
P
Et
P
PEt3
Ni
2
2
H
H
Si
Si
-PEt3
Me
Me
Si
Si
Me2
Si
2 Si
Ni
Me Si
2
P
Et2
2Si
P
Et2
+PEt3
H
H
6
6'
Scheme 4 Reversible SCSC transformations between 6 and 6’
1
5

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Figure 1 Thermal ellipsoid plot (50% probability level for Ni, P, Si, and C atoms) of
compound 6. Hydrogen atoms bound to carbon atoms are omitted for clarity
Figure 2 Thermal ellipsoid plot (50% probability level for Ni, P, Si, and C atoms) of
compound 6’. Hydrogen atoms bound to carbon atoms are omitted for clarity
1
6

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Table of Contents
Two unique silyl nickel complexes that generate two new Si-Si single bonds have
been prepared unprecedentedly, which exhibit reversible
single-crystal-to-single-crystal transformations upon removal and rebinding of the
coordinating PEt molecule.
3
-
PEt3
PEt3
+